Peel and chop the onion. Heat the oil in an ovenproof pan or roasting pan. Fry the onion in it. Sprinkle with sugar and melt briefly. Add sauerkraut and 2 teaspoons of paprika powder and steam for about 10 minutes.
Season with salt and pepper. Deglaze everything with cream and apple juice. Braise open in the preheated oven (electric cooker 200 °C/ circulating air: 175 °C/ gas: level 3) for 20-25 minutes. Stir crème fraîche and 1/2 teaspoon paprika powder until smooth.
Wash the fish, dab dry and cut into 4 pieces. Sprinkle with lemon juice. Season fish with salt and pepper. Place on the sauerkraut. Spread paprika-crème fraîche on the fish. Braise at the same temperature for another 15 minutes.
Garnish with parsley. Mashed potatoes taste good with it.