1 medium sweet onion such as Vidalia, quartered crosswise and broken into rings (discard small center rings)
2 tablespoons olive oil
Preheat oven to 450 degrees. In a food processor, pulse cornflakes and breadcrumbs until fine crumbs form, then transfer to a medium bowl. In another medium bowl, whisk together egg, buttermilk, flour, and cayenne and season with salt and pepper.
Dip onion rings in egg mixture (letting excess drip off) and dredge in cornflake mixture; place on a large plate. Pour oil onto a rimmed baking sheet. Place in oven and heat 2 minutes. Remove sheet from oven and tilt to coat evenly with oil. Arrange onion rings on sheet. Bake, turning once, until onion rings are golden brown, about 16 minutes. Season with salt.