Preheat your oven to 375°F (190°C).
In a large bowl, combine the sliced zucchinis, tomatoes, onion, and minced garlic. Drizzle with olive oil and sprinkle with oregano, basil, salt, and pepper. Toss until the vegetables are well coated.
In a greased 9x13 inch baking dish, layer half of the vegetable mixture evenly across the bottom.
Sprinkle half of the mozzarella and half of the Parmesan cheese over the first layer of vegetables.
Add the remaining vegetable mixture on top, followed by the rest of the mozzarella and Parmesan cheese.
Cover the baking dish with aluminum foil and bake for 30 minutes. Remove the foil and bake for an additional 15-20 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven and let it cool for 5 minutes. Garnish with fresh chopped basil before serving.