Peel the onions. Boil 4 litres of water, 1 onion, peppercorns and salt. Wash the fish thoroughly, dab dry, cut off the head, halve lengthwise and cut into slices (approx. 5 cm wide).
Leave slices in the salt water for about 2 minutes, remove from the water and drain well on kitchen paper. In the meantime, cut the remaining onions into cubes. Heat the oil in a pan and fry the onion cubes in it while turning them until they are glassy.
Mix sour cream, salt and pepper. Peel off the skin of the fish slices. Place the slices in an ovenproof dish, spread steamed onions on top, sprinkle with paprika and pour over the sour cream.
Bake the fish in a preheated oven (electric cooker: 175°C/ gas: level 2) for 25-30 minutes. In the meantime, knead flour, eggs, salt and approx. 50 ml water with the dough hooks of the hand mixer, form dumplings from the dough and cook in hot salt water for approx. 20 minutes.
Wash parsley, dab dry and chop finely. Remove the dumplings from the water, drain well, melt the fat in a pan and fry the dumplings briefly in it and sprinkle with parsley.
Garnish the fish with parsley to taste and serve some dumplings in the dish. Serve the rest of the gnocchi separately. Fresh salad tastes good with it.