Heat 1 teaspoon oil in a large nonstick skillet over medium-high heat. Add mushrooms and onion; sauté 5 minutes or until tender. Spoon into a large bowl; set aside. Heat remaining 1 teaspoon oil in pan. Add zucchini and squash; sauté 4 minutes or until tender. Add taco seasoning and garlic; cook 1 minute. Add to mushroom mixture in bowl; toss well. Wipe clean with a paper towel.
. Coat 1 side of a tortilla with cooking spray. Place tortilla, coated side down, in pan; top with 2 tablespoons cheese, 1/4 cup vegetable mixture, and an additional 2 tablespoons cheese. Fold tortilla in half; cook 3 minutes or until lightly browned. Turn quesadilla over; cook 1 minute or until lightly browned, pressing down on quesadilla with a spatula to flatten as it cooks. Remove from pan; keep warm. Repeat procedure with remaining tortillas, cheese, and vegetable mixture. Serve quesadillas immediately with salsa and sour cream.