Peel, wash and quarter the potatoes. Cook in boiling salted water for about 20 minutes, then drain well. In the meantime, wash the herbs. Chop basil, chives into small rolls.
Clean and wash the tomatoes and zucchini. Cut a tomato in quarters and remove the seeds. Cut the flesh into small cubes. Cut the remaining tomatoes and zucchini into thin slices. Cut mozzarella into slices as well.
Press the potatoes through a potato ricer. Mix with egg, flour and herbs. Season with salt and pepper. Heat 3 tablespoons of oil in a large pan. Add the potato mixture to 4 approx. 1 cm thick and 8 cm large roasts.
Fry over medium heat for about 3 minutes on each side. Then place on a baking tray lined with baking paper. Prepare the remaining oil and potato mixture in the same way. Alternately cover the roasts with prepared tomatoes, zucchini and mozzarella.
Season with salt and pepper. Sprinkle with tomato cubes. Bake in the preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: level 4) for 7-10 minutes. Serve garnished with basil. Serve with a colourful salad.