Mix ground beef, bread crumbs, onion, egg, salt, and pepper in a large bowl until well combined.

Shape the mixture into meatballs, about 1 1/2-inch in diameter.

Cook meatballs in a large nonstick skillet over medium heat until browned and cooked through.

In a medium bowl, mix brown sugar, water, ketchup, cider vinegar, soy sauce, and cornstarch until smooth.

Pour the sauce over the meatballs, reduce heat to low, and simmer until the sauce thickens, about 3 to 5 minutes.