Prepare the Onions: Peel the onions and slice them into 1 cm thick rings. Separate the rings gently, discarding the tiny inner ones or saving them for another dish. You’ll want only the large, sturdy rings for the best presentation.

Create the Marinade: In a shallow bowl, whisk together the buttermilk and egg until the mixture is pale yellow and completely smooth, with no streaks of egg white visible.

Soak the Rings: Submerge the onion rings in the buttermilk mixture, ensuring each one is fully coated. Cover the bowl and refrigerate for at least 20 minutes. This step tenderises the onion and helps the batter stick, creating that crisp shell.

Make the Dry Coating: In a separate, wide bowl, combine the plain flour, cornflour, baking powder, smoked paprika, garlic powder, cayenne (if using), salt, and pepper. Whisk thoroughly until the spices are evenly distributed throughout the flour; you should see a uniform, slightly peachy colour from the paprika.

Heat the Oil: Pour the vegetable oil into a large, heavy-bottomed pot or deep fryer. Heat it to 180°C (350°F). To test, drop a tiny pinch of flour into the oil; it should sizzle and float immediately without smoking.

Coat the Rings: Working with one ring at a time, lift it from the buttermilk, letting the excess drip off. Drop it into the flour mixture and toss gently with a fork until completely coated. Shake off any excess flour; you want a light, craggy coating, not a thick, pasty one. You should hear a soft rustling sound as you shake off the loose flour.

Fry in Batches: Carefully place 4-5 coated rings into the hot oil, ensuring you do not overcrowd the pan. Fry for 2-3 minutes, flipping once halfway through. The rings will turn a deep, golden brown and the sizzling will become more vigorous. When done, they will feel rigid and light when lifted with a slotted spoon.

Drain and Season: Transfer the cooked rings to a wire rack set over a baking tray. Immediately sprinkle with a tiny pinch of flaky sea salt while they are still hot. Do not use paper towels, as they will trap steam and make the rings soggy.