Preheat oven to 375 degrees F (190 degrees C). Lightly butter a 9x9-inch pan.
In a large bowl, combine Cheddar, Parmesan, oregano, basil, and garlic. Season with salt and pepper, and set aside.
Arrange half of the zucchini slices in the pan. Sprinkle 1/4 of the cheese and herb mixture on top. Arrange half of the tomatoes, and top with another 1/4 of the cheese mixture. Repeat layers.
Melt butter in a skillet over medium heat. Stir in onions, and cook until soft and translucent. Stir in breadcrumbs; cook until they have absorbed the butter. Sprinkle on top of casserole.
Cover loosely with foil, and bake in a preheated oven for 25 minutes. Remove foil, and bake until the top is crusty and the vegetables are tender, about 20 minutes.
