Cut potatoes into bite-sized pieces and soak in water. Mince the onion.

Boil potatoes in a pot with enough water to cover them. Simmer for 7 minutes until tender, then drain.

Mix tonkatsu sauce, ketchup, and mustard in a bowl to make the sauce.

Dry the pot, return the potatoes, and heat with salt until dry and fluffy. Mash while hot.

Heat salad oil in a pan, cook ground meat until browned, then add onion and cook until translucent. Add soy sauce, sugar, sake, nutmeg, and pepper.

Combine mashed potatoes, cooked meat mixture, and butter in a bowl. Mix well and chill in the fridge.

Divide the mixture into 10 portions, shape into balls, and coat with flour, beaten egg, and breadcrumbs.

Heat oil to 180°C and deep fry croquettes for 2-3 minutes until golden brown.