Wash the potatoes and cook in boiling salted water for about 20 minutes. Drain potatoes, rinse, peel and mash them finely with a potato masher while still hot. Let cool down a little. Knead potatoes, egg, flour and baking powder. Season to taste with salt, pepper and nutmeg.
Form 8 large dumplings from the potato mixture with floured hands and press them flat a little. Divide the onion mixture into eight pieces and add to the potato mixture. Form round with floured hands and place in a greased casserole dish. Bake in the preheated oven (electric cooker: 175 °C/ gas: level 2) for 1 hour. 10 minutes before the end of the cooking time, brush the potato patties with cream. Coarsely grate the cheese, sprinkle over the casserole and let it melt. Peel and finely dice the onion. Heat the fat in a saucepan. Sauté onion in it. Pour on the tomatoes and stock and let it boil down a little while stirring.
Peel and finely dice the onion. Heat the fat in a saucepan. Sauté onion in it. Pour on the tomatoes and stock and let it boil down a little while stirring. Serve with the tomato sauce. Garnish with parsley