Preheat oven to 375 degrees. Lightly spray a 9-inch square baking pan with non-stick spray.
You'll need to slice your vegetables on the thin side. Mine were about 1/4 inch thick, some may have been a little thicker.
Once you have your veggies prepped, combine the cheeses, garlic powder, and Italian seasoning in a small bowl.
In your prepared pan, place a layer of squash slices, add a layer of onion slices. Top that with tomato slices and then 1/4 more of the cheese mixture. Repeat with the squash, onion, cheese, tomatoes, and cheese. You will end with a layer of tomato and cheese.
Melt butter and mix in bread crumbs; sprinkle over the cheese layer.
Cover with aluminum foil and bake for 25 minutes. Remove foil and bake for another 20 minutes or until the top is golden brown and the veggies are cooked through. Let stand for a few minutes to set before slicing.