Preparation

1. Preheat the oil in the deep fryer to 180 °C (350 °F). Place a wire rack on a baking sheet or line it with several sheets of paper towel. Preheat the oven to 100 °C (200 °F) to keep the onion rings warm.
2. Cut the onion into 1-cm (½-inch) thick slices. Break them into rings and keep the small pieces for another use.
3. In a large bowl, combine the buttermilk and eggs. Set aside.
4. In another bowl, combine the flour, breadcrumbs, baking powder, and cayenne pepper.
5. Dip about a quarter of the onion rings in the buttermilk mixture and shake to remove any excess. Dredge in the breadcrumb mixture and coat thoroughly. The onions should be covered with a thin layer of breadcrumbs. Place on a baking sheet. Repeat with the remaining onion.
6. Fry a few slices at a time, for about 1 minute per side, or until golden brown. Drain on the prepared baking sheet. Season with salt. Keep warm. Repeat with the remaining slices. Serve with chipotle mayonnaise or in the Onion Ring and Chipotle Hamburgers (see recipe).