Preheat oven to 350 degrees. Grease bottom of a 9 x 13-inch baking dish with olive oil. Layer sliced zucchini into prepared baking dish, then cover with onions.
Pour tomato sauce over onions and zucchini. Sprinkle oregano, garlic salt, black pepper and bread crumbs, in that order, over sauce. Top with Mozzarella cheese then sprinkle with Parmesan cheese.
Cover and bake in preheated oven for 35 to 45 minutes. Makes 8 to 10 servings. Enjoy!