Trim excess fat from steak. Partially freeze steak; slice across grain into 2- x 1/4-inch strips.
Combine flour and salt. Dredge steak in flour mixture; sauté in hot oil in a large skillet until browned. Add mushrooms, beef broth, catsup, butter, Worcestershire sauce, and mustard; stir well. Cover and cook over low heat 1 hour. Just before serving, add sour cream; stir until well blended. Serve over hot buttered noodles.