Wash the fish fillet, cut into bite-sized cubes and drizzle with lemon juice. Heat the margarine in a saucepan. Briefly sweat the flour in it and deglaze with milk. Let the fish cubes simmer in the sauce for about 10-15 minutes.
Chop the cucumber and add it with the capers. Refine with cream. Season to taste with salt, pepper, dill and, if you like, a pinch of sugar.