1. Place a large nonstick skillet coated with cooking spray over medium-high heat. Add mushrooms and onion; cook 3 minutes or until tender. Remove from heat.
2. Wipe pan with paper towels; recoat with cooking spray. Coat 1 side of a tortilla with cooking spray. Place tortilla, coated side down, in pan, and top with 1/2 cup mushroom mixture, 2 tablespoons cheese, and a tortilla; cook 2 minutes or until browned. Coat top of quesadilla with cooking spray. Turn quesadilla using a wide spatula; cook 1. minute or until browned, pressing down on quesadilla to flatten as it cooks. Set aside and keep warm. Repeat procedure with remaining tortillas, mushroom mixture, and cheese.