In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, parsley, and egg. Add garlic powder, onion powder, salt, black pepper, and Worcestershire sauce. Mix until combined.
Shape the mixture into meatballs about 1.5 inches in diameter. You should have around 12-16 meatballs.
Heat olive oil in a skillet over medium heat. Brown the meatballs on all sides for about 5-7 minutes. Transfer to a slow cooker.
In the same skillet, sauté mushrooms and onions for 3-4 minutes. Add beef broth and bring to a simmer.
Pour the mushroom mixture over the meatballs in the slow cooker. Cook on low for 6-8 hours or high for 3-4 hours. Mix cornstarch and water to create a slurry and add to the slow cooker 30 minutes before serving.