In a medium bowl, whisk together the soup, broth, Worcestershire, garlic, and black pepper. Set aside.
Place the mushrooms and onions in the bottom of a 6-quart slow cooker. Add the frozen meatballs. Pour the soup mixture over top.
Cover and cook on LOW for about 4-6 hours or on HIGH for 2-3 hours.
Stir the sour cream into the slow cooker until it's completely blended into the sauce.
Serve the meatballs and sauce with noodles, if desired. Garnish with chopped fresh herbs and/or a dash of paprika.
