In a large pot boil the zucchini 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 zucchini, onion and garlic in a 9x12 inch casserole dish, and mix well. Pour the spaghetti 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.