Peel and wash the potatoes and cook them in boiling salted water for about 20 minutes. Rinse cod in cold water and dab dry. Cut crosswise into 4 large portion pieces. Wash, dry and slice the lemon. In a large saucepan, boil up about 2 l water, wine, lemon slices and 2 tsp salt. Let the fish pieces simmer at low heat for 18-20 minutes
Clean or peel and wash the vegetables. Cut vegetables into very thin strips (julienne). Heat butter and steam the vegetable strips for 3-5 minutes. Season with salt and pepper
Heat the base sauce, stir in the mustard and foam up briefly with a hand 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. Sprinkle with the herbs