In a mixing bowl, combine the ground beef, breadcrumbs, milk, chopped onion, egg, garlic powder, salt, pepper, and Worcestershire sauce. Mix until well combined.
Roll the mixture into meatballs, about 1-inch in diameter. You should have around 18 meatballs.
In a frying pan over medium heat, brown the meatballs on all sides for about 5-7 minutes. This step adds flavor but is optional if you prefer to skip it.
Transfer the browned meatballs into the slow cooker.
In the same frying pan, pour in the beef broth and ketchup, scraping the bottom of the pan to release any stuck bits. Bring to a simmer for a few minutes, then pour over the meatballs in the slow cooker.
Cover and cook on low for 6 hours or until the meatballs are cooked through and tender.
If you want to thicken the sauce, mix the cornstarch and water in a small bowl to form a slurry. Stir it into the sauce in the slow cooker and let it cook for an additional 15 minutes.
Serve the meatballs warm, spooning the sauce over mashed potatoes or rice for a complete meal.