Mince 2 garlic cloves. Cut remaining clove in half; set aside.
Place the mushrooms in a food processor; pulse 10 times or until finely chopped. Place minced garlic, parsley, and oil in a large nonstick skillet; cook 30 seconds over medium-high heat. Add mushrooms; cook 6 minutes or until liquid evaporates and mushrooms are tender, stirring constantly. Remove from heat; stir in salt and pepper. Cool slightly.
Lightly coat both sides of bread with cooking spray. Heat a grill pan over medium-high heat; add half of bread. Cook 1 minute on each side or until toasted. Rub 1 side of each bread slice with cut sides of garlic clove halves. Repeat procedure with remaining bread and garlic. Spoon about 2 tablespoons mushroom mixture over each bread slice, and top with about 2 teaspoons cheese.
Any combination of mushrooms will work--but the more varieties you use, the more memorable this appetizer will be. Try mixing exotic varieties with common ones for layered flavors. Prepare the topping up to a day in advance; reheat or serve at room temperature over toasted bread.