In large skillet over medium heat, melt butter; add onion. Cook 5 minutes or until tender. Add beef; cook until just done. Stir together bouillon, milk, flour, and Worcestershire; add to skillet with mushrooms. Heat to boiling; reduce heat to low. Meanwhile, cook noodles according to package directions; drain. Stir sour cream into beef mixture; spoon over hot noodles. Sprinkle with parsley, if desired. NOTE: Recipe can be halved.