To prevent the breading from falling off the onion rings, I pat the rings dry before bathing them in the marinade - too much moisture is the breading's enemy. And really give the onion rings a good dip in the foamy egg whites before you coat them. As well, breadcrumbs need a little pressure to stick onto the rings, so don't be gentle. That said, once the breadcrumbs are on the onion, handle with care in the air fryer so as not to disturb that fresh new coat.