2 large sweet onions
1 quart peanut oil
2 cups buttermilk
Kosher salt and freshly ground black pepper
4 cups all-purpose flour
1/2 teaspoon cayenne powder (optional)
Peel the onions and slice them crosswise into 1-inch-thick slices. Separate each slice into individual rings, and remove the slick thin membrane covering the inside of each ring.
In a heavy skillet over medium heat, or in a tabletop deep fryer, heat the oil to 375 degrees F and then line a baking sheet with paper towels and set aside.
Divide the flour between 2 large baking dishes and season each dish liberally with salt and pepper and 1/4 teaspoon of the cayenne (if using). Place the buttermilk in a large baking dish and season liberally with salt and pepper as well.
Work in batches, dredging some of the onion rings in one of the flour dishes and tap off the excess. Next, dip the rings in the buttermilk allowing the excess to drain off, and then dredge the rings in the second dish of flour, making sure to coat the rings evenly. Tap off any excess and transfer the batch of coated rings to the hot oil.
Fry the rings, turning once or twice, until golden brown and tender, about 4 minutes. Remove with a wire skimmer and drain on paper towel-lined baking sheet. Season immediately with salt. Repeat until all of the rings have been cooked. Serve hot with your favorite sauce or dip like Thai sweet chili sauce, horseradish cream, steak sauce or just plain ketchup.