To make the meatballs: Mix beef, bread crumbs, onion, egg, salt, and pepper together in a large bowl until well combined.
Shape mixture into 1 ½-inch diameter balls.
Cook meatballs in a large nonstick skillet over medium heat until evenly browned and cooked through.
To make the sauce: Mix brown sugar, water, ketchup, vinegar, soy sauce, and cornstarch together in a medium bowl until smooth.
Pour sauce over meatballs. Reduce heat to low and simmer until sauce is thick, 3 to 5 minutes.
Serve hot and enjoy!
