1 yellow onion – ends trimmed off, peeled, and cut crosswise into 1/2-inch slices
2 egg whites
2 tablespoons milk
1 pinch cayenne pepper, or to taste
3 cups panko bread crumbs, or more if needed
½ cup all-purpose flour
salt and ground black pepper to taste
Preheat oven to 450 degrees F (230 degrees C).
Separate onion slices into individual rings.
Whisk egg whites with milk in a bowl until thoroughly combined; season egg mixture with a pinch of cayenne pepper. Place panko crumbs into a separate bowl.
Place onion rings into a large resealable plastic bag. Add flour, salt, and black pepper; seal bag and shake until rings are well coated with flour.
Drop flour-coated onion rings into egg white-milk mixture, a few at a time, and toss lightly with tongs until coated. Place rings into panko crumbs and gently shake the bowl to toss the crumbs with the onion rings until rings are coated with crumbs.
Transfer coated onion rings to a large baking sheet; spray rings lightly with cooking spray.
Bake in the preheated oven until onion rings are tender and crumbs are lightly golden brown, 12 to 15 minutes.