Peel the potatoes and boil them in salted water until semi-cooked. Cut all of the vegetables into round slices.
Arrange the vegetables in a suitable oven dish that you've smeared with olive oil in the following order: tomatoes, potatoes, onions, until out of ingredients. Season with salt and sprinkle with olive oil.
Bake until golden. Before it's ready, add the mozzarella, cut into round slices. Bake another 2-3 min. until done.
Sprinkle with basil.