1. Add meatballs, mushrooms, onion, and garlic to a lightly greased 3 1/2- to 4-qt. slow cooker. Whisk together beef broth, tomato paste, and flour, whisking until smooth. Pour over meatball mixture.
2. Cover and cook on Low 6 to 7 hours. Stir in sour cream. Serve over hot cooked egg noodles.