Remove fat from steak.
Cut meat across the grain in strips and sprinkle with salt and pepper.
Melt butter in saucepan; add meat and saute quickly, tossing the strips around to brown evenly.
Add sliced onions and cook until transparent.
Add beef broth and bring to a boil.
Turn heat down.
Stir in mustard.
Cover and simmer 1 hour or till meat is tender.
Five minutes before serving add flour to thicken broth.
Add sour cream, wine and drained mushrooms.
Salt to taste and heat briefly.
Serve over steamed rice.