In a large skillet, sauté onion in olive oil until tender.
Add zucchini and cook until just tender, about 5 minutes.
Stir in garlic, salt, pepper, oregano, and basil. Cook for 1 minute.
Add diced tomatoes, tomato paste, and water. Bring to a boil.
Reduce heat and simmer uncovered for 10–15 minutes or until thickened.
Serve over hot cooked pasta and top with Parmesan cheese if desired.