Adjust oven rack to middle position, set a wire rack over a rimmed baking sheet, place the baking sheet in oven, and heat the oven to 250° F.
(Note: Keep liquid ingredients separate from dry ingredients)
With a fork, combine the buttermilk, hot sauce, and sugar in a medium bowl until the sugar dissolves.
Using a clean fork, combine the flour, baking powder, cayenne, salt, and pepper in a shallow baking dish.
Peel and cut the onion into 1/4” thick slices. Separate the onion slices into rings.
Dip the rings in the buttermilk and shake off the excess liquid. Dredge the rings in the flour mixture, shake off the excess, and gently place the rings on a second wire rack set over a rimmed baking sheet. Repeat until all the rings are coated. Overlap the rings if necessary.
Line a large plate with a double layer of paper towels. Meanwhile, heat oil to 400° F. (Measured with an instant-read thermometer) over medium-high heat in a large, 8-quart, heavy-bottomed Dutch oven with a diameter is approximately 12”.
Add the onions to form a single layer on the oil’s surface and cook until golden brown, no more that 3 minutes. (oil temperature should not slip below 375° F. You may need to adjust stove’s heat to maintain the temperature.) Using tongs, transfer the onions to the paper towel-lined plate. Let stand 2 minutes to drain, then carefully transfer to the rack in the warm oven. Replace the paper towels on the plate. Return the oil to 400°F. and fry the remaining onions, transferring them to the paper towel-lined plate to drain, then to the wire rack in the oven for each batch.
Serve onion rings hot from the oven.