In a large pot, boil the courgettes until tender; drain. Meanwhile, in a medium frying pan heat the olive oil over medium heat and saute the onion and garlic until the onion is tender.
Combine the courgettes, onion and garlic in a 20x30cm baking dish; mix well. Pour the pasta sauce over the mixture and stir well. Top with mozzarella cheese (use more or less depending on your preference).
Bake in preheated oven for about 20 minutes or until heated through and cheese is bubbly.