For the sauce, heat the butter and sweat the flour in it until lightly. Add the stock, wine and cream and bring to the boil while stirring, then simmer gently for about 10 minutes, stirring again and again. Season with mustard, salt, pepper and sugar (sugar can also be left out). Only stir in the chopped dill at the very end.
Cut the fish into 2 pieces, bring 0.5 l water with salt, bay leaf, a halved onion and 2 lemon wedges to the boil. Put the fish in and let it simmer for about 10 minutes over a mild heat.
Parsley potatoes and a fresh salad go well as a side dish.