Begin by preheating the oven to 450°F (230°C). Prepare a large, rimmed baking sheet by coating it evenly with cooking spray to prevent the onion rings from sticking and to encourage a golden crust.

In a medium-sized bowl, combine the egg whites, milk, and cayenne pepper. Whisk the ingredients together until fully incorporated, ensuring the mixture is smooth and slightly frothy.

Place the panko bread crumbs in a separate shallow bowl, setting up an efficient station for coating the onion rings.

Using tongs, dip each onion slice into the egg mixture, ensuring it is evenly coated. Allow any excess liquid to drip off back into the bowl. Then, transfer the slice to the panko crumbs, gently shaking the bowl or pressing the crumbs onto the ring to ensure full coverage.

Place each coated onion ring onto the prepared baking sheet, leaving a small space between each ring. Once all rings are arranged, lightly spray the tops with additional cooking spray to facilitate even browning.

Bake the coated onion rings in the preheated oven for approximately 12 to 15 minutes, or until the onions are tender and the panko coating is a light golden brown. Monitor closely to prevent over-browning.