In a large bowl, combine zucchini, tomatoes, onions, olive oil, thyme, oregano, garlic powder, black pepper, and salt. Toss until vegetables are evenly coated.
Spread the vegetable mixture in an even layer in a 9x13-inch baking dish.
Sprinkle the mozzarella cheese and Parmesan cheese evenly over the vegetables.
Bake for 25-30 minutes, or until the vegetables are tender and the cheese is melted and bubbly.
Serve hot.
Interesting Facts
The zucchini-tomato-and-onion casserole is a classic American dish.
The vegetables can be substituted with other vegetables such as mushrooms, bell peppers, or eggplant.
The vegetables can also be grilled or roasted for a different flavor.