Preheat the air-fryer to 400 F. Slice the onion into large rings. In a shallow dish, whisk egg and milk together. In another dish, whisk flour and seasoning together. Place the breadcrumbs into the last shallow dish.
Fully coat each onion ring into the egg mixture, then coat into the flour mixture, and back into the egg mixture. Coat fully in the breadcrumbs.
Place the onion rings into a large greased air-fryer basket. Air-fry in batches for 8-10 minutes.