Clean, wash and slice the carrot. Peel and slice the onion. Wash the fish. Bring lemon juice, 750 ml water, onion, carrot, peppercorns, salt and bay leaves to the boil, add fish slices and cook for 4-5 minutes.
Meanwhile, wash the lemon and cut into thin slices. Wash the dill, dab dry and pluck the flags from the stems. Carefully lift the fish slices out of the stock and keep them warm. Pour the stock through a sieve and collect it.
Measure out 375 ml of fish stock. Melt the fat in a saucepan, dust with flour, sauté and deglaze with the measured stock while stirring. Add cream and horseradish and bring to the boil while stirring.
Add wine, season with salt, pepper and sugar. Arrange the fish on four plates, spread some sauce over it and sprinkle with pepper. Serve lemon slices, dill pennants and garnish with gherkin if desired.
Add the rest of the sauce extra. Rice tastes good with it.