Cover the gutted and cleaned fish with water in a pot, add salt, bay leaf and plenty of vinegar. Bring everything to the boil briefly and then simmer on low heat for about 15 minutes.
Then remove the fish from the pot, bone it and cut it into pieces (size to your liking – preferably in bite-sized pieces). Let the fish pieces steep in the still hot broth! Heat the butter in the pot, add the flour and sweat it. Add enough of the fish stock, stirring constantly, to make a slightly creamy sauce when it comes to a boil.
Add the mustard and simmer on very low heat for about 10 minutes. Now add the cream or crème fraîche and chopped parsley and season with salt and pepper.
Serve with boiled potatoes and salad (e.g. cucumber salad).