What's not to like about savory, saucy beef?
Cut the fat and gristle from the roast, then cut it into strips 1/2 inch thick by 2 inches long. Season the strips with 1/2 teaspoon of both salt and pepper.
In a big skillet over medium heat, melt the butter and immediately brown the beef. Push the beef strips off to one side of the skillet; add the onions to the other side and cook slowly for 3 to 5 minutes. Push the cooked onions over to the beef side.
On the now-empty side of the pan, stir the flour into the leftover juices. Pour in the beef broth and bring to a boil, stirring constantly. Turn the heat down and stir in the mustard. Put a cover on and let the whole thing simmer for 1 hour or until the meat is tender.
Five minutes before you serve the beef, stir in the mushrooms, sour cream, and white wine. Let it heat briefly, then add salt and pepper to taste. Serve over noodles or rice.