Peel beetroot and potatoes. Cut 1/3 of the beetroot into small cubes, the rest and potatoes into pieces. Peel and finely dice the onion. Heat 2 tablespoons of fat in a saucepan. Fry the onion, potato and beetroot pieces in it.
Season with salt and pepper. Add stock, bring to the boil, cover and cook for about 20 minutes. Heat 1 tablespoon of fat in another pot. Sauté diced beetroot in it. Add 5 tablespoons of water, cover and cook over low heat for about 10 minutes until firm to the bite, remove from heat.
Wash the fish, dab dry and cut into the skin several times. Cut the fish into pieces, turn it lightly in flour and tap it off. Heat the oil in a large pan. Fry the fish fillets in it on the skin side for 4-5 minutes, depending on their thickness.
Turn the fish and continue to fry on the meat side for about 1 minute. Season with salt and pepper. Add horseradish to the beetroot-potato mixture and mash everything to a puree. Season to taste with salt, pepper, some sugar and horseradish.
Fold in the beetroot cubes. Wash parsley, dab dry, pluck leaves from the stalks and chop. Arrange everything and sprinkle with parsley.