Cut the fish into pieces, salt, pepper and shave with lemon juice. Let stand for 30 minutes in a cold place.
Cut the onion into half rings, grease the baking dish with butter and put the onion on the bottom, salt and pepper it.
Put the fish on top of the onion and bake at 190 degrees for 30 minutes.
Mix sour cream with flour, just salt and water the fish a little. Bake another 10 minutes.
Sprinkle fish with chopped dill before serving.