Preheat oven to 450°.
Heat oil in a large nonstick skillet over medium-high heat. Add the zucchini and mushrooms; sauté 10 minutes. Remove from heat. Add 1/4 cup cheese, parsley, salt, pepper, garlic, and tomatoes.
Combine 1/4 cup cheese and squash. Arrange squash mixture in a large gratin dish or shallow 1 1/2-quart baking dish coated with cooking spray. Spoon tomato mixture over squash. Combine 1/4 cup cheese and breadcrumbs; sprinkle over tomato mixture. Bake at 450° for 15 minutes or until bubbly.