Preheat the Oven:
Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper and lightly spray it with cooking spray.
Prepare the Onion Rings:
Peel the onions and slice them into 1/2-inch thick rings. Separate the rings and set them aside.
Set Up the Breading Stations:
In the first shallow bowl, add the flour and season with a pinch of salt and pepper.
In the second bowl, whisk together the eggs and milk.
In the third bowl, mix the panko, plain breadcrumbs, garlic powder, paprika, cayenne pepper, and a pinch of salt.
Bread the Onion Rings:
Dip each onion ring into the flour, shaking off the excess. Then, coat it in the egg mixture, allowing any excess to drip off. Finally, dredge it in the breadcrumb mixture, pressing gently to ensure an even coating. Place the coated rings on the prepared baking sheet in a single layer.
Bake the Onion Rings:
Lightly spray the breaded onion rings with cooking spray. Bake for 15–20 minutes, flipping halfway through, until the rings are golden brown and crispy.