Peel the potatoes and run under tap water. Drain well. Mix with soy sauce and sugar. The potatoes will absorb the flavour well.
Heat vegetable oil in a frying pan. Fry the chopped onion over medium heat.
After the onion is translucent, add the mince, salt, pepper and nutmeg.
Add the potatoes along with the seasonings. Add the sake and mirin. Cover and cook for 10 minutes.
Uncover the lid and mash the potatoes with a wooden spatula and let the excess moisture evaporate. When the potato mixture have thickened, turn off the heat and let cool.
Prepare the coating: flour, egg yolk mixed with water and fresh panko.
After shaping the potato mixture to your liking coat with flour, egg yolk water and fresh panko (in this order) all over.
Heat oil for deep-frying and check the temperature of the oil by dropping some panko into the oil. If they sink, then quickly rise to the surface, it is ready. Deep-fry the croquettes until golden brown.