sauté onion in a lightly greased large pan over medium heat two minutes. Add mushrooms, and sauté 2 to 3 minutes or until onion is tender.
Whisk together beef broth and next three ingredients in medium bowl until smooth. Add soup mixture and meatballs to onion mixture in pan. Cook meatballs according to time of package directions, stirring occasionally.
Meanwhile, prepare noodles according to package directions.
Remove meatballs mixture from heat; stir in sour cream until smooth and blended. Serve meatball mixture over hot cooked noodles. Garnish if desired. Mix 6 to 8 servings.