For the sauce, peel and dice the onion and beetroot. Heat the oil. Add vegetables and braise for about 4 minutes. Wash coriander, shake dry. Pour broth to the vegetables.
Add the coriander, except for some garnish, bring to the boil and simmer for about 10 minutes. Puree with the cutting stick of the hand mixer. Pour the sauce mixture through a sieve into a pot. In the meantime, wash the fish, pat dry, cut into portions and season with salt and pepper.
Heat the lard in a pan. First fry the fish on its skin side for about 3 minutes, then turn and fry for a further 2 minutes. Bring the sauce to the boil. Add crème fraîche and foam the sauce with the hand mixer's cutting stick, adding butter piece by piece.
Arrange sauce and fish on plates. Garnish with coriander and lemon.