Peel the onion and divide it into quarters, place in a large saucepan with all the other ingredients for the stock and bring to the boil to a strong stock. Reduce the heat and add the prepared fish fillets and cook over a low heat for 10 to 15 minutes. Lift out the fish and keep warm.
Melt the butter in a second saucepan, add the flour and stir until smooth. Deglaze with a little fish stock and then gradually fill up the stock with a ladle and stir until a creamy sauce is formed. Add capers and season with salt, sugar and pepper. Wash and finely chop the dill, add to the sauce.
Arrange the fish on plates, cover with the sauce and serve with boiled potatoes. In addition, green salad or cucumber salad go very well with it.
Attention: the whole brew is not used for the sauce! Dispose of the rest.