Preheat oven to 375F. In skillet, heat 1 T oil over medium heat. Add onions; saute 10 minutes, or until tender. Add 1 t thyme and 1/4 t each salt and pepper.
Cut each tomato lengthwise into 4 slices. Slice zucchini crosswise into 1/2-inch diagonal slices. Arrange tomato, zucchini and onion slices in rows across width of 2 1/2 qt shallow baking dish.
In small bowl, combine remaining 3 T oil, 1/2 t salt and 1/4 t pepper; mix. Drizzle over vegetables. Cover tightly with foil. Bake 15 minutes; uncover. Bake 30 minutes or until vegetables are tender; let stand 10 minutes before serving.
* Tian is the name of both a layered vegetable mixture and the baking dish it's cooked in, Provence, Franc.