Preheat oven to 450. Line a baking sheet with foil and spray heavily with cooking spray.
In a bowl whisk together egg whites and milk. Place breadcrumbs in a second bowl. Add flour, salt and pepper to taste, cayenne pepper, and garlic powder in a large zip lock bag, seal and shake to mix seasonings.
Open the bag and add the onion rings. Seal and shake to coat the rings well. Take onion rings out and one at a time, dip into egg-milk mixture making sure to coat all sides, then into the breadcrumbs, also coating all sides.
Place rings in a single layer on prepared baking sheet. Spray heavily with cooking spray. Bake 20-25 minutes (depending on how crispy you want them).
While onion rings are baking, prepare the dipping sauce. Mix together mayo, ketchup, and hot chili sauce. Chill until ready to serve.