1. If using onions, sauté them until they are soft.
2. Cut the tomatoes and add them to the onions, then simmer with herbs de Provence, salt, and pepper.
3. Slice the zucchini and add them to the mixture.
4. Add tomato sauce (or tomato puree as a substitute) and 1 teaspoon of powdered sugar (more in winter when the tomatoes are less flavorful).
5. Let the mixture simmer for about 20 minutes over medium heat.
6. Once the mixture is ready, layer it in a large dish:
7. 1 layer of lasagna noodles, 1 layer of the mixture, 1 layer of béchamel sauce, 1 layer of gruyère cheese, and so on (adding a little salt each time), with a generous amount of gruyère cheese on the top layer.
8. Bake in a hot oven (375°F or 200°C) for 35 minutes.
9. Serve when it is nicely browned and bubbly.