01 - Combine ground meat, breadcrumbs, Parmesan, egg, milk, parsley, salt, black pepper, garlic powder, and onion powder in a large mixing bowl. Fold gently until just incorporated.
02 - Form the mixture into uniform 1 1/2-inch meatballs and place evenly on a baking sheet.
03 - For added flavor, broil the meatballs on high for 3 to 5 minutes until lightly browned on the exterior.
04 - Place the browned or raw meatballs into the vessel of a slow cooker.
05 - In a separate bowl, whisk together apple cider, apple cider vinegar, brown sugar, Dijon mustard, and soy sauce until smooth. Pour the glaze mixture over the meatballs in the slow cooker.
06 - Cover and cook on low heat for 3 to 4 hours or until meatballs reach an internal temperature of 160°F and are fully cooked.
07 - Mix cornstarch and water in a small bowl to create a slurry. Stir slurry into slow cooker approximately 15 minutes prior to serving. Cover and continue cooking for 10 to 15 minutes until the glaze is thickened.
08 - Plate meatballs and spoon glaze over each portion. Optionally, garnish with additional fresh chopped parsley.