Separate the onions into individual rings. Use only the larger rings for this recipe; reserve the smaller rings for another use. You should have 20 rings total. Lay the rings in a single layer in a large rectangular baking dish. Pour the milk over the rings and allow them to soak for about 20 minutes, turning them once so that all surfaces of the onion rings have been exposed to the milk.
Preheat the oven to 425˚F. Place wire baking racks on each of two foil- lined baking sheets, and set them aside.
Put the flour in a shallow dish. Put the panko in a small dish.In a large bowl, whip the egg whites with a whisk until they are extremely foamy but not quite holding peaks.