Russian
Season the beef strips and sear quickly in the oil over high heat until browned, then remove.
Melt the butter and cook the onion until soft, then add the mushrooms and fry until golden.
Sprinkle over the flour and stir for a minute, then pour in the stock and bring to a simmer.
Lower the heat and stir in the sour cream and mustard to make a smooth sauce.
Return the beef with any juices and warm through for 2 to 3 minutes without boiling.
Season to taste and serve over rice, buttered noodles or mashed potato.