In a Dutch oven, combine spaghetti sauce and pepper flakes. Heat through over medium heat, stirring occasionally; keep warm.
Cut peppers in half. Remove and discard stems and seeds. Grill peppers and onion, covered, over medium heat 8-10 minutes on each side or until tender. Slice peppers. Stir onion and peppers into sauce; heat through.
Grill sausages, covered, over medium heat for 10-12 minutes or until heated through, turning occasionally. Place buns on baking sheets, cut side up. Top with cheese. Broil 3-4 in. from the heat 2-3 minutes or until cheese is melted. Top with sausages and pepper mixture.