In a mixing bowl, combine the ground beef, breadcrumbs, chopped onion, egg, garlic powder, salt, black pepper, Worcestershire sauce, and fresh parsley. Mix until well combined.
Form the mixture into meatballs, about 1 inch in diameter (you should have around 24 meatballs).
Heat a frying pan over medium heat and lightly brown the meatballs on all sides for about 5 minutes.
In the slow cooker, combine the beef broth, cream of mushroom soup, Worcestershire sauce, garlic powder, onion powder, salt, and pepper. Stir well to combine.
Gently place the meatballs into the gravy mixture in the slow cooker, ensuring they are well coated.
Cover the slow cooker and cook on low for 6 hours, or until the meatballs are cooked through and tender.
If you prefer a thicker gravy, mix the cornstarch with 2 tablespoons of water to create a slurry. Add this to the slow cooker during the last 30 minutes of cooking, stirring well.
Serve the Salisbury steak meatballs over mashed potatoes or rice for a complete meal.