Step 1

Cook fish and potatoes if they're not already cooked. Break the fish up into flakes.  Peel and cube the cooked potatoes.

Step 2

Melt butter in a saucepan, add finely chopped onion and cook until translucent (but not browned).  Sprinkle flour over the butter/onion mixture, and allow it to cook for a few seconds until it begins to "bloom" or bubble vigorously in the oil.

Step 3

Quickly add the milk or cream, and whisk until no lumps remain.

Step 4

Allow the mixture to simmer for a few minutes until it begins to thicken.

Step 5

Add the fish, potatoes, salt, and pepper.  Heat until everything is warmed through, and then remove from heat.

Step 6

Stir to break up the fish and potatoes into smaller pieces, if you choose, or not.

Step 7

Serve with rye bread, or with sliced onions and cucumbers.