Heat oven to 425°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray. Arrange onion pieces in baking dish. Bake 10 minutes. Add bell peppers and mushrooms to onions; spray with cooking spray and sprinkle with salt. Bake 15 minutes. Stir in tomatoes. Bake about 5 minutes longer or until vegetables are tender.