Coat both sides of each bread slice with cooking spray; rub both sides of each slice with cut side of garlic clove. Set garlic aside. Cut bread into cubes. Place cubes on a baking sheet; bake at 375° for 8 to 10 minutes or until crisp and browned.
Combine vinegar and next 3 ingredients in a jar; cover tightly, and shake vigorously. Rub inside of a wooden bowl with cut side of garlic clove. Add lettuce and mushrooms; add vinegar mixture to salad, and toss gently to coat. Sprinkle with cheese and pepper.