In a large skillet, heat 2 tablespoons butter over medium heat. Add onions, peppers and pepper sauce; cook and stir 5 minutes or until tender; remove and keep warm.
Place bun bottoms on a baking sheet, cut side up. Remove steak from freezer; cut into thin slices. Sprinkle with salt and pepper. In same skillet, heat remaining butter over medium-high heat. Add beef in batches; cook and stir 1-2 minutes or until meat is no longer pink. Remove from pan.
Layer bun bottoms with meat, onion mixture and cheese. Broil 4 in. from heat for 2-3 minutes or until cheese is melted. Replace tops.