• Remove any fat and gristle from the meat and cut into strips 1/2 inch thick by 2 inches long.
• Season with salt and pepper.
• In a large skillet 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 broth and water and bring to a boil, stirring constantly.
• Lower the heat and stir in mustard and mushrooms.
• Cover and simmer for 1 hour or until the meat is tender.
• Five minutes before serving, stir in the sour cream, and white wine.
• Heat briefly then salt and pepper to taste.