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 pot, boil up about 2 l water, wine, lemon slices and 2 teaspoons of salt. Let the fish pieces simmer for 18-20 minutes at low heat.
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.