Separate the sliced white onions into distinct rings and place them into a shallow bowl or baking dish.

Pour the buttermilk over the top, gently pushing the rings down to submerge them while you prepare the remaining ingredients.

In a small bowl, whisk together the mayonnaise, ketchup, prepared horseradish, paprika, and ground oregano.

Season the sauce with the kosher salt, coarse black pepper, and hot pepper sauce to taste.

Stir until smooth, then cover and refrigerate until ready to serve.

Place 2 cups of all-purpose flour into a shallow baking dish for dredging.

Whisk the chili powder into the flour and season generously with salt and pepper.

In a separate, large shallow bowl, whisk together 1 cup of all-purpose flour and the 1 cup of rice flour for the batter.

Gently stir the club soda and the bottle of beer into the flour mixture until just combined. (Do not overmix).

Pour about 3 inches of vegetable oil into a large, deep heavy-bottomed pot or Dutch oven (the oil should come no higher than one-third up the side of the pot).

Attach a candy thermometer to the side of the pot and heat the oil over medium-high heat until it reaches exactly 350°F.

Lift an onion ring out of the buttermilk and gently shake off the excess liquid.

Dredge the ring thoroughly into the seasoned dry flour mixture.

Dip the floured ring into the wet beer batter, allowing any excess batter to drip off.

Carefully slip the battered onion ring into the hot oil.

Repeat this process to fry several rings at a time, taking care not to overcrowd the pot so they can swim freely.

Use a slotted spoon or spider strainer to gently turn the rings in the oil until they are puffed and golden brown on both sides, about 3 to 5 minutes.

Transfer the fried rings to a paper-towel-lined baking sheet and immediately sprinkle with the chopped fresh parsley and flaky sea salt while hot.

Wait for the oil temperature to return to 350°F before frying the next batch.

Serve immediately alongside the spicy dipping sauce.