Cover the gutted and cleaned fish in a saucepan with water, add salt, bay leaf and plenty of vinegar. Bring everything to the boil briefly and then let it simmer over a low flame for about 15 minutes.
Then take the fish out of the pan, debone and cut into pieces (size as desired - preferably into bite-sized pieces). Let the pieces of fish soak in the still hot brew!
Heat the butter in the pan, add the flour and sweat. Add enough of the fish stock, stirring constantly, to create a slightly creamy sauce after it has boiled. Add the mustard and simmer on a very low flame 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 lettuce (e.g. cucumber salad).