For the zucchini au gratin, cut the zucchini into 1/2 cm slices. Heat the oil in a frying pan and fry the zucchini slices until light brown, remove and set aside.
Heat the butter in the pan and fry the onion until light brown. Add the finely chopped bell pepper and the tomato sauce, season with sugar, salt and pepper and simmer for 2-3 minutes.
Pour the sauce into an oblong dish and place the zucchini slices on top. Sprinkle with mozzarella cubes and grated Parmesan cheese.
Bake in a preheated oven at 200 °C until light brown. Serve with rice or boiled potatoes.