Add 1 diced onion, 3 cloves garlic, 2½ cups mushrooms, 1 bag meatballs, 2 tsp worcestershire sauce, 2¾ cups broth, and seasonings to a large slow cooker.
Stir until evenly mixed.
Cook on high for 3-4 hours.
Separate meatballs from the broth.
Add broth to a pot and bring to a boil.
Whisk together cornstarch and cold water.
Add to boiling broth and whisk to combine.
Once thick, add 2 Tbsp sour cream and stir to combine. Add meatballs back in.
Add meatballs back in.
Serve with pasta, rice, or mashed potatoes.