Preheat oven to 350 degrees. Grease bottom of a 9-by-13-inch baking dish with olive oil.
Layer slice zucchini into prepared baking dish, then cover with onion.
Pour tomato sauce over onion. Sprinkle oregano, garlic salt, black pepper and bread crumbs 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!