1. Preheat oven to 450°F Cover 2 baking sheets with aluminum foil; spray evenly with cooking spray.
2. Separate onion slices into rings. Dip into milk, then into the flour, turning to evenly coat both sides of each onion ring. Dip coated onion rings in egg whites, then into cornflake crumbs, again turning to evenly coat both sides of each onion ring. Place in single layer on prepared baking sheets; spray evenly with cooking spray.
3. Bake 12 to 15 minutes, or until onions are tender and golden brown.