1. Spray 10-inch nonstick skillet with cooking spray; heat over medium heat. cook onions in skillet about 5 minutes, stirring frequently, until tender. Stir in beef; heat through.
2. Set oven control to broil. Place rolls cut side up on cookie sheet; brush with dressing. Broil with tops 4 to 5 inches from heat for 2 minutes.
3. Divide beef mixture among rolls. Top with cheese. Broil about 1 minutes or until cheese is melted. Serve immediately.