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

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. 

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

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

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

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

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