First, soften the vegetables in butter, then add the wine, vinegar and water, and simmer for 10 minutes. Next place the fish into a dish and pour the vegetables and the sauce over the fish. Bring to a simmer, then remove from the heat. Drain off the liquid, reduce a little and stir in some butter. Next add the vegetables back into the sauce. Serve the fish on top of roast potatoes and dill, with the sauce and vegetables spooned round it.
