Coat a nonstick sauté pan with nonstick cooking spray over medium heat. Add frozen chicken and cook 5 to 7 minutes to heat through and slightly brown. Remove chicken from pan and set aside.
Add olive oil to same pan. Stir in mushrooms and cook 5 to 7 minutes or until lightly browned; pan will be fairly dry. Stir in garlic and cook 1 to 2 more minutes. Season with pepper.
Heat oven to 450 degrees F. Line 2 baking sheets with nonstick foil. Place a tortilla on each prepared sheet. Add one fourth of the cheese to each tortilla, then layer each with half the chicken and mushrooms. Divide remaining cheese between tortillas. Cover with the last 2 tortillas and brush lightly with olive oil. Bake at 450 degrees F for 7 minutes or until slightly golden-brown and cheese has melted.
Cut each quesadilla into 6 wedges. Serve with sour cream and chipotle salsa, if desired.