Put some water into a pot and bring it to the boil. First add the potatoes and then the fish so that they will all be cooked at the same time. Prepare the sauce: finely chop the onion and sauté it in butter until soft. Add some of the cooking water from the pot. Pour out the excess water from the pot and serve the potatoes and fish with warm butter and onion sauce. Freshly baked rye bread will crown this delicious meal.