Preheat oven to 350°.
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.
Combine mushrooms and balsamic vinegar in a shallow dish, turning mushrooms to coat. Let stand 15 minutes.
Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add mushrooms and vinegar; cook 5 minutes or until mushrooms are tender, stirring occasionally.
Spread roasted garlic pulp on 1 side of each bread slice; divide mushrooms evenly among bread. Spoon 2 teaspoons cheese onto each crostini. Place crostini on a baking sheet; broil 2 minutes or until toasted. Serve immediately.