Heat oven to 400° F. Place onion in a food processor or grate by hand.
In a large bowl and using your hands, combine beef, onion, garlic, mozzarella, Parmesan, egg, and spice, Form into 16 meatballs.* Dipping your hand into a bowl of water before rolling them will make the meatball surface smooth. Transfer the meatballs to a lightly oiled parchment-lined rimmed baking sheet.
Bake in the oven for 20-25 minutes, rotating the pan half way through.
Air Fryer
Preheat fryer to 400° F.
Place meatballs in a single layer. Reduce fryer to 380° F.