In large saucepan, combine meatballs and spaghetti sauce. Cook over medium heat for 10 to 12 minutes or until meatballs are hot.
Meanwhile, spray medium skillet with nonstick cooking spray. Add bell peppers and onion, cook over medium high heat for 5 to 6 minutes or until peppers are crisp tender, stirring frequently.
Split each bun lengthwise, cutting to but not through the bottom; place on ungreased cookie sheet. Put meatballs in buns, top with pepper mixture and cheese. Broil 4 to 6 inches from heat for 2 to 3 minutes or until cheese is melted.