For cod in mustard sauce, peel the potatoes, cut into quarters and boil in salted water until soft.
Bring the fish stock and white wine to the boil and reduce by half, then add the whipped cream and bring to the boil again a little.
Bone the fish fillets, season with salt and steam in a saucepan with a sieve insert.
Drain the boiled potatoes, drain and sauté in a frying pan with butter, sprinkle with parsley.
Season the sauce with mustard, add the cold butter and season with salt and pepper. Blend briefly before serving.
Serve the cod in mustard sauce with the parsley potatoes!