Cut the spring onion into small rings. Mix well with the mince, egg, panko, sesame oil, sake, sugar, soy sauce and, if desired, the ginger together in a bowl. Season with salt and pepper and form small balls. Heat oil in a pan and fry the meatballs well in it.
Meanwhile, stir water, ketchup, sugar, vinegar, soy sauce, sake, mirin and sesame oil into a sauce and briefly bring it to the boil. Mix a little flour with water. Stir into the sauce over low heat until it thickens.
If desired, add some sesame seeds and spring onions to garnish.