Fish stew (leftover fish in white sauce) - Plokkfiskur

I recently dined at an upscale restaurant in Reykjavík, Lækjarbrekka. Due to its location, in the very heart of the old town, it caters to many tourists and some of the menu items are quintessentially Icelandic. Imagine my surprise when I discovered that good old leftover food, plokkfiskur, on the menu. It's served au gratin with the classic accompaniment of rye bread and potatoes on the side.

There are jokes about plokkfiskur - it can be either a delicacy or a disaster. During the old days, when fish was served (in some homes) five days a week, this was the standard way of using up leftovers. If you didn't finish the fish at lunch, this was what you could expect to be served for dinner.

about 700 g cooked fish
about 500 g cooked potatoes
50 g margarine or butter
50 g flour
750 ml milk
1/4 tsp ground pepper
1/2 to 1 medium sized onion, optional

Any kind of cooked fish can be used, but to make this authentic, use cod, salt cod, haddock, or halibut.
Remove all skin and bones from the fish and flake with a fork. Cut the potatoes into small pieces.

Now make white sauce: Melt the margarine/butter over medium heat. Stir the flour into it, until smooth and thick. Continue stirring and add a small amount of milk. When the mixture boils, add more milk. Repeat this process until all the milk is used up. When the sauce is ready, add the fish and potatoes and warm through.
Most cooks add some onion to get more flavour. Chop it finely and fry it until it is soft but not browned and cook along with the sauce.

Very good with buttered rye or pumpernickel bread on the side.