In 2-quart saucepan, heat cooking sauce and meatballs over medium heat about 15 minutes, stirring occasionally, until thoroughly heated.
Place each bun on serving plate. Fill each with 6 meatballs; spoon 2 to 3 tablespoons sauce over meatballs. Sprinkle with 1/4 cup cheese. Top with bell pepper and onion. Serve with remaining sauce as dipping sauce.