Chop the onion. Cut carrots, potatoes and celery into small cubes, leek and celery thinly slices and cook everything in the fish stock for about 12 minutes.
Drizzle the prepared, washed and boned fish with lemon juice, season with salt, cut into pieces and add to the vegetables in the fish stock. Let the fish simmer for a further 12 minutes at a low temperature. Whisk egg yolks with sweet and sour cream and half a cup of fish stock and add to the soup, but do not let it boil anymore. Season with salt and pepper, sprinkle with chopped parsley and serve.