Wash the potatoes and cook in salted water for about 20 minutes. Quench cold. Peel and slice the potatoes. Clean and wash spring onions and cut into rings. Heat oil in a pan and fry the spring onions at medium heat for about 3 minutes.
Deglaze with broth. Season to taste with vinegar, salt, pepper and 25 g horseradish. Pour over the potatoes and mix well. Wash the carp and cut into 8 slices. Cook in boiling salted water for about 8 minutes.
Remove the carp tranches and carefully peel off the skin. Heat the fat in a pan. Dust the carp with flour and fry in the hot fat on each side for about 1 minute. Whip the cream with the whisk of the hand mixer until stiff and fold in the remaining horseradish.
Season potato salad again and arrange on plates with the carp. Put the horseradish cream in a piping bag with a star-shaped spout and sprinkle one tuff on each carp. Garnish the potato salad as desired with lamb's lettuce and tomato quarters.