1. Cook Morningstar Farms® Grillers Prime® Veggie Burger according to package directions. Immediately top with cheese. Let stand about 1 minute or until cheese melts.
2. Meanwhile, in small nonstick skillet heat oil. Add mushrooms and onion. Cook, stirring frequently, over medium heat for 3 to 5 minutes or until tender.
3. Place lettuce leaf, tomato and veggie burger on bun bottom. Top with mushroom mixture and bun top.