Take a small pan and add 200 ml dashi stock, 5 tsp Japanese soy sauce (koikuchi shoyu) and 4 tsp mirin. Boil for 1 minute, add ½ tbsp sugar to the pan and mix until dissolved.
Take the pan off the heat, pour the dipping sauce into a heatproof jug and add 1 dried shiitake mushroom. Leave to cool, then once cool to the touch, chill in the fridge for 20-30 minutes.
Cook 120 g dry soba noodles according to the instructions on the packaging (usually 5-6 minutes).
Once cooked, pour the soba noodles into a strainer and drain away the cooking water. Rinse under cold water and add a few ice cubes to chill them quickly.
Divide 1 tsp wasabi paste, 1 tsp grated ginger root, 2 tbsp tempura flakes (tenkasu) and 2 tbsp finely chopped green onions between small individual serving plates.
Place the soba noodles on the "zaru" and sprinkle 2 tbsp kizami nori (shredded nori) over the top. Serve each portion with individual cups of dipping sauce and the plate of additional ingredients to mix in.
You can use the same recipe to make zaru udon, just replace the udon noodles with soba noodles.
If you're doubling or tripling the recipe, the mentsuyu will take longer to cool.
You can keep left over mentsuyu in the fridge for about 1 month so I recommend making extra if you plan to make zaru udon or zaru soba often during the summer!