1. Melt the butter and oil in a pan until hot, fry the onion, add the mushrooms and raise the heat until cooked.
2. Add the steak and flash fry for a few minutes until the meat has browned, but be careful not to overcook it. Season and then add the vodka to the hot pan – take care because this will start the flaming process.
3. Once the flames have reduced, season well and add the sour cream and mustard. Cook until the cream is hot and serve with mashed potatoes.