In a large nonstick skillet coated with cooking spray, heat 1 tablespoon butter over medium-high heat. Add mushrooms and onion; cook and stir 4-5 minutes or until tender. Remove from pan. Wipe skillet clean., Place one slice cheese on each of four bread slices. Top with mushroom mixture, bacon and the remaining cheese and bread. Lightly spread outsides of sandwiches with remaining butter., In same skillet, toast sandwiches in batches over medium heat 45-60 seconds on each side or until golden brown and cheese is melted.