In a large mixing bowl, combine ground beef, ground pork, breadcrumbs, Parmesan cheese, parsley, egg, minced garlic, onion powder, salt, and black pepper. Mix until evenly combined.
Form the mixture into meatballs, approximately 1 to 1.5 inches in diameter.
Optionally, bake the meatballs at 400°F (200°C) on a baking sheet for about 15 minutes before adding them to the slow cooker.
In the slow cooker, combine beef broth, chopped onion, soy sauce, Worcestershire sauce, garlic powder, dried thyme, salt, and pepper. Stir to combine.
Carefully place the meatballs into the slow cooker, making sure they are submerged in the gravy.
Cover and cook on low for 6 hours.
In the last 30 minutes of cooking, mix cornstarch with water in a small bowl until smooth, then stir it into the gravy to thicken.
After cooking, taste and adjust seasoning if necessary before serving.