Remove any fat and gristle from the steak and cut into strips 1cm thick by 5cm long. Season with both salt and pepper.
In a large frypan over medium heat, melt the butter and brown the beef strips quickly, then push the beef strips off to one side. Add the onions and cook slowly for 3 to 5 minutes, then push to the side with the beef strips.
Stir the flour into the juices on the empty side of the pan. Pour in beef stock and bring to a boil, stirring constantly. Lower the heat and stir in mustard. Cover and simmer for 1 hour or until the meat is tender.
Five minutes before serving, stir in the mushrooms, sour cream and white wine. Heat briefly then salt and pepper to taste.