Cut the beef fillet into thin strips. Dice the onions. Slice the mushrooms thinly. Cut the pickled cucumbers into strips.
Heat the margarine in a large skillet over medium-high heat. Fry the beef strips in batches, about 2 to 3 minutes per batch until browned. Season each batch with salt and white pepper, then remove and set aside.
In the same skillet, sauté the onions and mushrooms over medium-high heat for 3 to 4 minutes until softened.
Sprinkle the flour over the vegetables and sauté for 1 minute, stirring.
Pour in the meat stock and bring to a boil.
Remove from heat and stir in the sour cream and mustard.
Add the beef and cucumber strips back to the skillet. Season with salt, white pepper, and a pinch of sugar. Stir to combine.
Garnish with parsley and serve with boiled potatoes.