Wash and clean the zucchini and slice them into thin oblong slices. Sort the spinach, wash and drain. Heat 2 tablespoons of oil in a large frying pan, fry the zucchini slices for about 2 minutes, turning them over. Put the spinach in the hot pan and let it collapse for a short time.
Roughly grind the mozzarella. Mix ricotta and half of the mozzarella, season with salt and pepper.
Onions peel, halve and finely dice. Heat 2 tablespoons of oil in a saucepan, sauté the onions for about 1 minute, dust with sugar and deglaze with vinegar and tomatoes. Season with salt and pepper and simmer for about 4 minutes.
Spread the zucchini slices with the ricotta mixture, cover with spinach and roll up. Put the tomato sauce in an ovenproof dish, spread the zucchini rolls in it and add the remaining mozzarella. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 15 minutes.
Wash basil, shake dry, pluck leaves from the stalks. Remove the casserole dish from the oven and sprinkle with basil and coarse pepper.