Clean, wash and cut the leek into rings. Fish wash, dab dry, cut into 12 small slices. Sprinkle with lemon juice. Heat 20 g fat in a large pot, fry leek briefly while turning. Season with salt and pepper, deglaze with 100 ml vegetable stock, bring to the boil, cover and cook for about 8 minutes.
Meanwhile melt 30 g fat in a pot, dust with flour and sauté while stirring. Gradually deglaze with 300 ml vegetable stock, milk and wine, stirring continuously. Add mustard. Bring to the boil while stirring and simmer for 2-3 minutes. Season to taste with salt, pepper and sugar. Keep warm
Heat clarified butter in a large frying pan, fry the fish slices for about 1 1/2 minutes on each side, season with salt and pepper. Wash parsley, shake dry, put some leaves aside for garnishing. Cut remaining parsley into fine strips. Sprinkle fish, leek and sauce with parsley and arrange parsley leaves on plates. Mashed potatoes taste good with it