Prepare the Meatball Mixture: In a large bowl, combine the ground chicken, panko breadcrumbs, chopped green onions, minced garlic, minced ginger, soy sauce, sake, mirin, beaten egg, salt, and pepper.

Shape the Meatballs: Using your hands, form the chicken mixture into small balls or oval shapes. If you are grilling, thread the meatballs onto skewers. If pan-frying, keep them separate.

Cook the Meatballs: If grilling, preheat your grill to medium heat and grill the skewered meatballs for 10-12 minutes, turning occasionally until fully cooked and browned. If pan-frying, heat a tablespoon of oil in a skillet over medium heat and cook the meatballs for 8-10 minutes, turning to ensure they brown evenly on all sides.

Prepare the Glaze: In a petite saucepan, mix together the soy sauce, mirin, sake, and sugar until well blended. Bring the mixture to a simmer over medium heat and cook until the sauce thickens slightly, about 3-4 minutes.

Glaze the Meatballs: Once the meatballs are cooked through, brush or drizzle the glaze over them, coating each meatball generously. If desired, you can grill or pan-fry the meatballs for an additional 1-2 minutes to caramelize the glaze slightly.

Serve: Serve the Tsukune hot, garnished with extra green onions if desired. These can be enjoyed as an appetizer or served with rice and vegetables for a complete meal.