Prepare the steak. Slice these into strips about 1cm wide. Season the meat with salt and pepper and set it aside for a few minutes.
Heat the butter in a large frying pan. Add the onion and saute for 2 minutes, then add the mushrooms and continue to cook until both are soft. Stir the paprika and mustard into the pan, coating the onion and mushrooms.
Pour the stock into the pan, then leave it to simmer until the liquid has reduced by about half. Stir in the crème fraiche/sour cream and set the pan aside for a few minutes.
In another large frying pan, heat the vegetable oil. When it’s smoking hot, add the strips of beef. Fry, stirring continuously, until the meat is browned on all sides. This should take about a minute at the most.
Reheat the onion and mushroom sauce, then add the beef. Addd more salt and pepper to your taste. Sprinkle with parsley and serve with tagliatelle, mash potato or fries.