Rinse fish cold and pat dry. Cut lemon into slices. Boil up plenty of water with lemon slices, bay leaves, peppercorns and salt. Add the fish and let it simmer at low heat for about 25 minutes.
Peel and wash the potatoes. Cook in boiling salted water for about 20 minutes. Clean and wash the leek. Peel carrots. Cut both into very fine strips. Melt the fat and toss the vegetables in it for about 6 minutes.
Season with salt and white pepper. Whip the cream until semi-solid and stir in the horseradish. Wash parsley, dab dry and chop finely, except for a little to garnish. Drain potatoes and sprinkle with chopped parsley.
Lift the fish out of the stock and drain well. Arrange on a plate with the vegetables and potatoes. Add horseradish cream.