In a large mixing bowl, combine the chicken mince, finely chopped spring onions, beaten egg, grated ginger, bread crumbs, and optional shiso leaves. Knead the mixture until pale and smooth, about 3-4 minutes.

Scoop out portions of about 1-2 tablespoons each and shape them into round meatballs. Use wet hands to prevent sticking and insert a skewer through each meatball for easy handling.

Heat vegetable oil in a large frying pan over medium-high heat. Once shimmering, add the meatballs and fry for 2-3 minutes on each side until golden brown.

Remove excess oil, leaving about a tablespoon. In the same pan, mix soy sauce, cooking sake, and sugar. Bring to a boil for about a minute.

Lower the heat, place meatballs back in the pan, and baste with sauce for another minute until the sauce thickens.

Remove the meatballs, arrange on a plate, and garnish with sesame seeds or chopped green onions if desired.