Wash the fish and dab dry. Sprinkle with lemon juice, salt and leave to stand. Bring plenty of salted water to the boil in a large pot. Season with halved onions, bay leaves and 1 stalk of dill, simmer for 10 minutes.
Let the fish chops simmer for about 10 minutes at medium heat. In the meantime melt butter. Sweat mustard seeds and flour in it. Add cream and 375 ml fish stock while stirring constantly, bring to the boil briefly.
Stir in mustard and simmer for another 5 minutes at low heat. Season sauce again with salt, pepper and sugar. Chop 1 stalk of dill, also stir in. Serve fish chops and mustard sauce in portions.
Boiled potatoes taste good with it.