Slice ½ inch off the top (stem end) of the onion, leaving the root end intact. Set the onion root-side down and slice straight down through the onion, starting ½ inch from the root, working around to create 12–16 even wedges.

Turn the onion cut-side up and gently spread the petals outward into a flower shape. Submerge in a bowl of ice water for 10 minutes to 1 hour, then lift out and pat lightly (just enough to remove drips).

Whisk 2 cups flour with the cayenne, smoked paprika, garlic powder, onion powder, black pepper, and salt in a wide bowl. In a second bowl, whisk the buttermilk, eggs, and remaining ½ cup flour until smooth and thick.

Dredge the onion in the seasoned flour first, working it between every petal. Shake off excess, dunk fully into the buttermilk batter working it between the petals, let excess drip off, then dredge once more in the seasoned flour.

Heat 3 inches of oil in a large pot to 350°F. Lower the onion in cut-side down using a spider strainer. Fry 3–4 minutes, flip root-side down, and fry another 3–4 minutes until deep golden all over.

Lift the onion onto a wire rack set over a baking sheet and let rest 2 minutes. Meanwhile, whisk together the mayonnaise, ketchup, horseradish, cayenne, and paprika for the sauce.

Gently spread the petals open wider, place the sauce in the center where the core was, and serve immediately.