1. In a large saucepan heat oil over medium heat. Add sweet pepper and onion; cook 4 to 5 minutes or until crisp-tender, stirring occasionally. Stir in meatballs, marinara sauce, and crushed red pepper. Bring to boiling; reduce heat. Simmer, covered, 15 minutes or until meatballs are heated through.
2. Cut a thin slice from to of buns; remove some bread, leaving about 1/2-inch-thick sides and bottom. Place meatball filling in buns top with cheese. Broil sandwiches 4 to 5 inches from the heat 1 to 2 minutes or until buns are toasted and cheese is melted.