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.
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. Cut each quesadilla into 4 wedges. Serve immediately with salsa, cilantro, and sour cream.