Increase heat to medium-high. In same skillet, cook beef 5 to 7 minutes, stirring frequently, until no longer pink; drain. Stir in 1 cup of the beef broth, the Worcestershire sauce, salt and pepper; heat to boiling. Beat flour and remaining 1/2 cup beef broth with whisk until incorporated; stir into beef mixture. Add mushroom mixture; return to boiling, and stir constantly about 1 minute or until mixture thickens. Remove from heat; stir in sour cream.