Clean, wash and cut the leek into pieces. Steam in 300 ml boiling salted water for about 12 minutes, drain and collect the cooking water. Melt the fat and sweat the flour in it.
Add cream and cooking water while stirring constantly, bring to the boil. Stir in mustard. Season with salt, pepper and lemon juice. Warm the leek in it. Wash the fish fillet. Season salt water with bay leaf and vinegar, bring to the boil briefly.
Let the fish fillets simmer for 8-10 minutes. Serve the fish, leek and sauce sprinkled with chervil leaves and pink berries. Garnish with lemon.