Mince the onion finely.

Heat salted butter in a pan and sauté ground beef and onion over medium heat until browned.

Add sugar, mirin, and soy sauce to the pan and cook until the liquid evaporates.

Peel and cut potatoes into 2cm cubes.

Boil potatoes in water for 5 minutes, drain, and mash roughly while still hot.

Mix mashed potatoes with the beef mixture, season with salt and pepper, and divide into 6 portions.

Shape each portion into oval patties and let them cool. Coat with flour, beaten egg, and breadcrumbs.