Begin by preheating your oven to 375°F (190°C). Grease a medium-sized baking dish lightly with olive oil to ensure your casserole doesn't stick.
Wash and slice the zucchini, yellow squash, Italian tomatoes, and red onions into uniform ¼-inch rounds. The even thickness ensures even cooking.
Take your vegetables and layer them in the prepared baking dish, alternating between zucchini, yellow squash, tomatoes, and red onions. The combination of colors will create a visually appealing presentation.
Cover the baking dish with foil and place it in the preheated oven. Allow the casserole to roast for approximately 20 minutes. This will give the vegetables time to become tender and infused with the delicious seasoning.
Baked casserole out of the oven, in a baking dish top view
After 20 minutes, remove the foil and sprinkle the grated cheddar and parmesan cheeses over the top. Pop the dish back into the oven for an additional 10 minutes, or until the cheese is melted and golden brown.
Serve and enjoy: Once your roasted zucchini and summer squash casserole is cooked to perfection, take it out of the oven and let it cool slightly. Serve it as a side dish to complement your main course