1. Cook veggie burger according to package directions. Top hot burger with cheese slice. Let stand for 1 minute or until cheese melts.
2. Meanwhile, in small nonstick skillet coated with cooking spray, cook sliced mushroom and onion rings over medium-low heat about 5 minutes or until tender, stirring frequently. Stir in steak sauce.
3. Serve burger topped with mushroom mixture between bread pieces.