Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a mixing bowl, combine ground beef, breadcrumbs, chopped green onions, egg, minced garlic, salt, and black pepper. Mix well until all ingredients are combined.
Shape the mixture into meatballs, about 1 to 1.5 inches in diameter, and place them on the prepared baking sheet.
Bake the meatballs in the preheated oven for about 15 minutes or until they are cooked through and browned.
While the meatballs are baking, prepare the sauce. In a skillet, combine soy sauce, brown sugar, water, cornstarch, sesame oil, minced ginger, and rice vinegar.
Heat the sauce over medium heat, whisking constantly until it thickens (about 5 minutes).
Once the meatballs are done baking, add and toss them into the skillet with the sauce. Allow them to simmer for an additional 2-3 minutes, ensuring they are thoroughly coated.
Serve hot, garnished with additional green onions if desired. Enjoy!