Melt 30 g unsalted butter in a frying pan over a medium-low heat. Sauté 170 g onion (diced) until soft and translucent, without letting it brown.
Stir in 110 g crab meat and 1 tsp tomato ketchup.
Sift in 4 tbsp all-purpose flour and mix until evenly distributed.
Add 200 ml whole milk one-third at a time while whisking continuously.
Add ¼ tsp salt, 1 pinch ground black pepper, 1 pinch nutmeg powder and 2 tbsp preferred shredded melting cheese.
Switch to a spatula and keep mixing until the filling has thickened to the consistency of mashed potato.
Transfer to a wide container and leave to cool. Once cool enough to touch, place in the freezer for 30 minutes or until firm enough to shape.
Shaping and frying
Preheat your cooking oil to 170 °C (338 °F) to 180 °C (356 °F). While you wait, make a batter by mixing 50 ml cold water, 1 egg and 5 tbsp all-purpose flour together in a bowl until smooth.
Prepare two plates, one with flour for dusting and one with 75 g panko breadcrumbs.
Take the crab filling from the freezer and divide into equal portions (the base recipe makes 8).
Roll each piece into a barrel shape.
When the oil is ready, coat each barrel in plain flour, then batter, then a generous coating of panko breadcrumbs. Once coated, place immediately into the hot oil.
Fry them in batches to avoid overcrowding and lowering the oil temperature. Keep uncoated croquettes in the freezer while you're waiting for the others to cook. Fry until golden all over.
Rest on a wire rack to drain excess oil.
Mix 2 tbsp tomato ketchup and 1 tbsp Worcestershire sauce together in a bowl to make the korokke sauce.
Serve and enjoy!
Notes
Never skip the chill: Hot sauce will not shape, and underchilling leaves water that flashes to steam in the oil and bursts. The freezer sets it in about 30 minutes, or chill it 1 to 3 hours in the fridge.Do not add too much moisture: The wildest variable is the canning liquid. Separate the crab meat from it and do not add it back. If the sauce feels loose, work it longer to cook off the water before you chill it.Coat evenly, and do not touch it once frying: Thin or patchy spots are where it bursts. Use a batter for an even, thick coat, hold 170-180°C (338-356°F), do not crowd the pot, and leave it until the coat sets.Shape it into a log: The filling is soft, and a flat oval patty cannot support it. A cornerless log rolls in the oil so the whole surface sets evenly and resists splitting.Use real crab if you can: Canned is fine, but reach for real crab over imitation. That sweet aroma the moment it hits your tongue only comes from the real thing.