Coat nonstick medium skillet with cooking spray. Heat over medium-high heat. Add onion and bell peppers. Cook, stirring constantly, 2 to 3 minutes, adding a little water from time to time to prevent sticking and overbrowning. Preheat broiler. Divide sliced flank steak over opened rolls. Cover with onion mixtu re. Top with cheese. Broil, close to heat source, 1 minute or until cheese is melted and bubbly.