1. Preheat oven to 350°.
2. Remove white papery skin from garlic heads (do not peel or separate cloves). Wrap each head separately in foil. Bake at 350° for 1 hour; let cool 10 minutes. Separate cloves; squeeze to extract garlic pulp. Discard skins. Set pulp aside.
3. Combine mushrooms and balsamic vinegar in a shallow dish, turning mushrooms to coat. Let stand 15 minutes.
4. Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add mushroom and vinegar mixture; cook 5 minutes or until mushrooms are tender, stirring occasionally.
5. Preheat broiler.
6. Spread roasted garlic pulp on 1 side of each bread slice; divide mushrooms evenly among bread slices. Spoon 2 teaspoons cheese onto each crostino. Place crostini on a baking sheet; broil 2 minutes or until toasted. Serve immediately.