Grate the onion into a small bowl instead of chopping it so it blends into the mixture and helps keep the meatballs juicy.
In a large bowl, combine the ground turkey, egg, breadcrumbs, grated onion, garlic, Italian seasoning, salt, pepper, and Parmesan if using.
Mix gently with your hands just until combined so the meatballs stay tender.
Roll the mixture into 16 meatballs, about 1 1/2 inches each, for even cooking.
Preheat the air fryer to 380°F for 2 to 3 minutes if your model benefits from preheating.
Spray the basket lightly with oil, then place the meatballs in a single layer with space between them.
Air fry at 380°F for 9 to 12 minutes, shaking the basket or turning the meatballs around the 5-minute mark.
Cook until the tops are browned, the juices run clear, and the internal temperature reaches 165°F.
Rest the meatballs for 3 minutes before serving.
Serve them plain for a crisp exterior, dip them in marinara or tzatziki, or toss them in warm marinara right before serving.