Partially freeze beef. Thinly slice beef across the grain into bite-size strips. In a 10-inch skillet melt margarine or butter; add onions and pepper. Cover and cook over medium-low heat about 10 minutes or until tender, stirring occasionally.
Remove onion-pepper mixture from skillet with a slotted spoon. If necessary, add additional margarine to skillet. Add beef; cook and stir over medium-high heat for 2 to 3 minutes or until done.
To serve, spread rolls open face on a baking sheet. Divide beef and onion-pepper mixture among rolls. Top with cheese. Broil 4 to 5 inches from the heat for 1 to 2 minutes or until cheese is melted. Serve immediately. Makes 4 servings.