Melt the margarine in a saucepan and sauté the chopped onions and cucumber cubes. Season with salt, pepper and garlic. Add the diced potatoes and tomato paste. Cut the fish fillet into large pieces, lightly salt and season with paprika powder. Also add to the pot. Deglaze with broth and let the fish fillet simmer in it.
Mix the cream with the mustard and the chopped dill. Finally, undergo. Serve hot.
Tip: Serve with lettuce and small black bread wedges that have been roasted in hot oil with a little grated garlic.