Peel and wash the potatoes and cook them in boiling salted water for 20-25 minutes. Rinse fish cold and pat dry. Cut crosswise into 4 large portion pieces. Wash, dry and slice the lemon. In a large saucepan, bring approx. 2 l water, wine, lemon slices and 2 tsp salt to the boil. Let the fish pieces simmer for 18-20 minutes at low heat
Melt 25 g butter in a saucepan, sprinkle flour over it and deglaze with stock while stirring. Simmer for 2-3 minutes, stirring continuously. Stir in cream and mustard, season with salt and pepper and season to taste with a few drops of lemon juice. Keep warm
Wash the herbs, dab dry, pluck off flags or leaves and chop finely. Peel and wash carrots. Clean and wash the leek. Cut vegetables into very thin strips (julienne). Heat 2 tablespoons of butter and steam the vegetable strips for 3-5 minutes. Season with salt and pepper
Briefly foam the mustard sauce with the blender. Drain the potatoes. Remove fish from the stock with a skimmer, drain well. Arrange a piece of fish with vegetables, potatoes and mustard sauce. Serve sprinkled with herbs