Vegetable Tempura Udon Soup

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Such a beautiful dish! I really enjoyed it.

★★★★★
– Kelly

Udon does not need shrimp on top. Walk into an udon shop in Japan and you will see burdock, kakiage, and whatever vegetables are in season riding on the bowl with the same quiet confidence.

Perilla leaves take a few seconds. Shiitake mushrooms need a little longer. Eggplant goes in last. Fry them in the same oil, in that order, and the batter starts to make sense.

Udon noodles in simple broth in a brown bowl next to a variety of vegetable tempura including shiitake mushrooms, asparagus, shiso leaves and eggplant

Vegetable Tempura Udon

Recipe Snapshot

What is it? Vegetable tempura udon joins 2 separate dishes, plain udon in broth and just-fried vegetable tempura, served together to make one.
Flavor profile: A clear katsuobushi and kombu broth with soy sauce, mirin, and a sharp salt edge. The coating breaks lightly over sweet steamed vegetables. Soaked in broth, it becomes another food.
Why this recipe works: The flour, water, and egg go in cold, so the batter stays thin, not heavy. I stop mixing while small lumps remain, which keeps the coating from setting tough.
How to make it: A 30 minute chill, then cut and flour the vegetables, mix the batter, and fry from the leaves down. The broth simmers in a second pot. No special equipment.
What to expect: 15 minutes of prep, 15 minutes of frying, and a separate 30 minute chill. About 1 hour in all, and it serves 4. The stumble is over-mixing the batter.

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What is Vegetable Tempura Udon?

Vegetable tempura udon is a Japanese noodle dish that sits thick wheat noodles in a dashi-based kake udon broth and lays battered, deep-fried vegetables on top or on the side. Once the hot broth reaches the coating, it stops making any sound. Whether the tempura goes on the noodles or waits on a plate beside them is yours to decide.

Tempura Udon Ingredients

What You’ll Need for Veg Tempura Udon

Ingredients needed to make vegetable tempura udon on a white background with labels. From top to bottom, left to right: shiitake mushrooms, dashi stock, eggplant, asparagus, salt, perilla leaves, udon noodles, soy sauce, sake, mirin, shichimi togarashi.

Vegetables: Take your pick, whatever is easy to find. What I am frying here is perilla leaves (shiso), shiitake mushrooms, eggplant and asparagus, but the dish does not stand or fall on those 4. Choose by what is in the shop that day.
Udon noodles: I personally recommend frozen udon over dried ones. Sanuki udon from the freezer aisle of an Asian grocery is plenty. Frozen over chilled, over dried.
Dashi stock: The broth is decided here. Use an awase dashi made from katsuobushi and kombu, or a store-bought dashi packet if that is easier. Only if you reach for granulated dashi, watch the salt across the whole pot, since the granules carry plenty of it along with flavor boosters.

What You’ll Need for Tempura Batter

Cake flour: The batter wants a low-protein flour, and on English shelves that is cake flour. What to check on the bag is whether it has been bleached. Chlorinated cake flour has been changed into something that holds onto its water, and while a piece fries the oil moves in before that water can leave. So pick the bag marked unbleached.

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Substitution Ideas

Katsuobushi dashi → A kombu and dried shiitake shojin dashi: Leave the egg out of the tempura batter and build the dashi from kombu and dried shiitake instead, and you have a bowl with no fish and no egg in it. The glutamate in the kombu and the guanylate in the dried shiitake meet, so the umami does not thin out.
Egg → Mayonnaise, or nothing at all: Mayonnaise carries emulsified oil in fine drops that cut the water in the batter, and its vinegar holds gluten back, so it works less as a replacement and more as a push toward a lighter coating. Mayonnaise is itself made of egg, so if egg is the thing you are leaving out, the option to take is the empty one.
Tempura → Kakiage: Kakiage is a fritter of mixed vegetables bound in batter, so cut them thin, hold them together with dusting flour, bring the batter in last, and drop the whole clump into the oil.
The batter you would throw out → Tenkasu: Instead of discarding the bits of batter that break off while you fry, keep them and scatter them over the bowl as tenkasu, crunchy fried batter bits. They stand closer to a seasoning than a garnish, because the oil they hold dissolves into the broth while the bits themselves drink it up and thicken it. My tenkasu recipe covers how to make them.

How to Make My Vegetable Tempura Udon

Before you start (Mise en place):

Put the water and the carbonated water in the fridge and chill them for 30 minutes. An unopened bottle of carbonated water is better. The egg stays in the fridge until the last moment too.
Sift the cake flour and the cornstarch together, and chill the whole bowl in the freezer for 30 minutes.
Wash, peel and cut the vegetables. Cut the eggplant into wedges and soak them in cold water for 5 to 10 minutes.
Wipe every piece with paper towels. No water left on the surface is the signal that you can move on. The eggplant sat in water, so give it the most care here.

Still water in a jug and sparkling water in a sealed bottle

Sifting dry ingredients for tempura batter into a mixing bowl

Cut eggplant for tempura soaking in a bowl of water

Equipment Notes

To develop this recipe, I used a tempura pot with a built-in thermometer.

Japanese tempura frying pot with wire draining rack lid on a white background

STEP

Heat the oil and flour the vegetables

i. Start heating the frying oil toward 180°C (356°F).

ii. Dust the wiped vegetables with a thin coat of flour and line them up in the order they go into the pot, perilla leaves first, then shiitake, then asparagus, and eggplant last.

Why the vegetables get dusted with flour first

Water left on the surface turns to steam the instant it hits the oil, shoving the batter aside and spitting. A thin coat of flour drinks that water and gives the batter something to hold. Wiping and flouring are not 2 jobs but the 2 halves of 1. The wetter the eggplant, the more it shows.

STEP

Mix the batter at the last minute

i. Combine the chilled water and the carbonated water, crack in the cold egg, and beat it through. If foam rises, lift it off with a spoon and throw it away.

scooping foam out of eggs to make tempura batter

ii. Sift in the chilled flour in 3 additions, cutting crosses through it with chopsticks or a spatula.

whisking dry ingredients into egg mixture with chopsticks to make tempura batter

iii. Stop the moment small lumps sit scattered through it. Put the bowl back in the fridge until the oil is ready.

tempura batter in mixing bowl with wooden chopsticks

Why I stop while small lumps are still scattered through it

What I watch is the size of the lumps. Small ones dotted through the bowl is my stop, because lumps that small mean the flour and water have not joined yet. A few big ones and I have not mixed enough. Once it goes smooth they have joined, and the batter will not fry light.

STEP

Fry in order, from the leaves to the eggplant

i. Read the oil with a drop of batter. Sinking partway and springing back up means 180°C (356°F). Scattering across the surface means too hot, so lower the heat and wait. Sitting on the bottom means too cold. Then fry the perilla leaves, battered on 1 side only, for a few seconds.

shiso Perilla leaf with one side coated with tempura batter frying in a pot of oil

ii. Follow with the shiitake, asparagus, and eggplant, each dipped in batter on the way in. Frying each kind in its own batch keeps the cooking even.

Asparagus tempura frying in a Japanese-style tempura frying pot

frying vegetable tempura (shiitake mushrooms and eggplants) in a Japanese style fryer

iii. Stand the pieces on a rack, not stacked, so the oil runs off.

3 pieces of asparagus tempura on a wire rack

How to read the moment a piece is ready

4 things happen at once. The bubbles go small and quiet, the frying sound climbs from low to high and light, the coating floats, and it sets while short of color. Vegetables differ, so read the signs. That last sign is the hardest to catch, and color keeps moving on the rack, so golden is too late.

STEP

Boil the udon and build the broth

i. Boil the udon for the time on the package, drain it, then rinse it quickly under fresh boiling water.

partially cooked udon noodles in a strainer over a bowl

ii. In another pot bring the dashi to a boil, add the soy sauce and mirin, and let it go about 2 minutes. Turn off the heat before you add the salt.

Why the noodles get rinsed before they go into the bowl

Starch that did not dissolve into the cooking water stays on the surface of boiled noodles. Put them straight into the bowl and the broth clouds, and the edges of the flavor go soft. Rinsing takes that slick off, and fresh boiling water does it without cooling the noodles down.

STEP

Assemble and eat it hot

i. Put the noodles in a bowl, pour the hot broth over them, and lay the kamaboko fish cake slices on top (optional).

ii. Set the tempura on the bowl or on a plate beside it. Scatter chopped green onions over it, and add shichimi togarashi if you like.

Serving size

If you follow the default recipe, it will yield 5 servings.

Tips for the Best Japanese Tempura Udon

Dry every piece of vegetable before you flour it. A coating that slides off and oil that spits back at you are both failures you cannot undo once the frying is under way, and both are settled before the piece ever meets the oil. Wipe each piece down and dust it thinly, treating the 2 as 1 job, and that is where both of them get closed out.
Cover no more than half the surface of the oil at a time. Crowd the pot and the temperature falls, and in cooler oil the coating drinks instead of setting. Everything you added to save yourself a trip takes longer and comes out heavy for it. 2 rounds gets you there sooner.
Fry the tempura at the last minute, never in advance. From the second a piece leaves the oil, the hot water inside turns to steam and moves back into the coating. That is why tempura fried ahead goes soft on the plate, and it has nothing to do with skill. Keep the run from the pot to the bowl short, with the noodle water and the broth pot standing ready before you start.

Tempura is a dish where the shorter the trip from the oil to the bowl, the better it goes. Set the noodle water and the broth pot up to go on the heat before the first piece drops, and nothing you fry has to wait around.

Storage & Meal Prep

Fridge: Broth stores well for a few days.

Freezer: Cool the tempura fully, freeze in 1 layer, wrap and freeze for up to a month.

Meal prep: Broth only, a few days chilled. Measure and chill batter ingredients ahead of time, but never mix it in advance. Noodles should be cooked fresh.

Reheating: Tempura, 5 minutes at 200°C (392°F) in a toaster oven, air fryer or oven. Avoid microwaving, which will make it soggy.

What to Serve With Tempura Udon Soup

Veggie Tempura Udon Troubleshooting

Why did my tempura come out heavy instead of crisp?

It is 1 of 3 things. The batter was overmixed, the batter or its ingredients had gone lukewarm, or too much went into the pot at once and the oil temperature fell. If no dry flour was left and the surface had turned smooth, that batter was overmixed and it will not come back, so stop earlier on the next one. Take the flour, water and egg straight from the fridge, and cover no more than half the oil at a time, and while a single round will not tell you which cause was at work, fixing all 3 brings the coating back light.

Why did the coating slide off in the oil?

Water was still sitting on the surface of the vegetable. It flashes into steam the instant it reaches the oil, and the force of that pushes the coating away. Dry each piece with paper towels, dust it with a thin coat of flour, then take it through the batter, and the flour gives the batter a footing to hold. The wetter the vegetable, the bigger the difference it makes.

Why did my udon go soft before I finished eating?

The noodles sat in hot broth too long. They keep drinking it up, so they carry on softening after they reach the bowl. Hold the noodles and the broth apart and put them together right before you eat. If a portion has already gone soft, tip it into the pot of broth and finish it as nikomi udon, meaning udon simmered in the broth itself, and it eats well that way.