Wash the potatoes thoroughly and cook in plenty of boiling water for about 20 minutes. Drain the potatoes, rinse and drain. Peel the potatoes and press them hot through a potato ricer. Season with salt and nutmeg. Add semolina, flour, butter and egg, knead briefly with the dough hooks of the hand mixer (if the mixture is too moist, knead in spoonfuls of flour)
Form 6-8 dumplings from the potato mixture with floured hands and place them in a large pot with plenty of boiling salted water. Let them simmer for about 15 minutes at low to medium heat (simmering gently) (If they float on top, they are done)
Wash the meat, dab dry. Heat 1 tablespoon of oil in a frying pan. Fry the meat for about 10 minutes, turning it over, season with salt and pepper, remove and let it rest
Remove the dumplings, let them drip off a little, place them on a damp tea towel and let them cool down. Wash, clean and slice the zucchini and tomatoes. Cut meat and dumplings into slices. Mix cream, 200 ml water and vegetable stock, bring to the boil. Mix starch with a little water, thicken the simmering broth with it, simmer for 1-2 minutes, season with salt, pepper and 1 pinch of sugar. Layer meat, dumplings, tomatoes and zucchini alternately in a greased casserole dish (approx. 1.8 litres capacity), pour cream mixture over them. Grate cheese, sprinkle over it
Bake in the preheated oven (electric range: 225 °C/ convection oven: 200 °C/ gas: level 4) for 10-15 minutes. Remove the gratin, let it cool down a little, garnish with basil