In a large bowl, combine ground beef, breadcrumbs, chopped onion, milk, egg, garlic powder, Worcestershire sauce, salt, and black pepper. Mix until well combined.
Form the mixture into meatballs, about 1.5 inches in diameter.
Place the meatballs in the slow cooker.
In a separate bowl, whisk together cream of mushroom soup, beef broth, soy sauce, ketchup, and dried thyme. Pour the mixture over the meatballs in the slow cooker.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the meatballs are cooked through and tender.
If you prefer a thicker sauce, mix cornstarch with water in a small bowl to create a slurry. Stir it into the sauce in the slow cooker during the last 30 minutes of cooking.
Serve the meatballs over mashed potatoes or egg noodles, spooning the sauce over the top.