01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - In a large bowl, blend ground meat, egg, breadcrumbs, green onions, garlic, ginger, soy sauce, sesame oil, salt, and black pepper using clean hands or a spatula until just combined.
03 - Form mixture into 20 to 24 evenly-sized meatballs and arrange on the prepared baking sheet.
04 - Bake for 18 to 20 minutes or until meatballs are golden and cooked through.
05 - While meatballs bake, combine soy sauce, mirin, brown sugar, honey, rice vinegar, ginger, and garlic in a small saucepan. Simmer over medium heat.
06 - Mix cornstarch with water in a small bowl to create a slurry. Gradually stir into the simmering sauce and continue cooking, stirring, until thickened, approximately 2 minutes. Remove from heat.
07 - Transfer baked meatballs to a skillet or wok. Pour over teriyaki sauce and gently toss to coat. Heat for 2 to 3 minutes until meatballs are sticky and glossy.
08 - Plate meatballs, garnish with sesame seeds and sliced green onions as desired. Serve hot.