In a large bowl, combine the 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, and place them in the slow cooker.
In a separate bowl, whisk together the 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 to 8 hours or on high for 3 to 4 hours, until the meatballs are cooked through and tender.
Once cooked, serve the meatballs over mashed potatoes or egg noodles, and garnish with fresh parsley.