Chop the onions finely. Cut the potatoes, zucchini and tomatoes into round slices 1/4″ (0.5 cm) thick.
Braise the onions in some of the oil until it becomes transparent - about 5 min., while stirring constantly. Pour out the onions in an oven dish.
Arrange the vegetable slices upright in the following order: slice of potato, slice of tomato, slice of zucchini.
Sprinkle them with salt, oregano and basil and the remaining oil.
Cover with foil and bake 35 min. at 356°F (180 °C). Remove the foil, sprinkle with grated cheese and bake another 30 min.