In a large bowl, combine the ground beef, breadcrumbs, green onions, egg, garlic, ginger, black pepper, and salt. Mix until well combined.
Form the mixture into meatballs, about 1 inch in diameter. You should have approximately 20 meatballs.
In a large skillet, heat the vegetable oil over medium heat. Add the meatballs and cook for about 5-7 minutes, turning occasionally, until browned on all sides.
While the meatballs are cooking, in a small bowl, whisk together the soy sauce, brown sugar, cornstarch, sesame oil, and rice vinegar until smooth.
Once the meatballs are browned, pour the sauce over them in the skillet. Stir gently to coat the meatballs in the sauce.
Reduce the heat to low and let the meatballs simmer in the sauce for an additional 5 minutes, allowing the sauce to thicken.
Serve the meatballs warm, garnished with sesame seeds and additional chopped green onions if desired.