Wash the beetroot and cook in the bowl in boiling salted water for about 45 minutes. Rinse in cold water and let cool down a little, peel. Dice half of the beetroot, cut the rest into sticks.
Sauté the peeled and diced onion together with the beetroot cubes in a tablespoon of butter. Add the vegetable stock and simmer for a few minutes, then puree. Add crème fraîche, bring to the boil briefly, season with horseradish, salt and pepper.
Pat pikeperch fillets dry, season with salt and pepper, turn in flour. Heat clarified butter in a pan, fry pikeperch fillets briefly on the meat side, then fry on the skin side for about 5 minutes.
In the meantime, fry the beetroot sticks in a tablespoon of butter and season with salt and pepper.
Serve the fish on preheated plates with the sauce and the beetroot sticks.