Peel the onions and slice them very thinly into rings or half-moons (about 1/8 inch / 3 mm). Separate the slices into individual rings and place them in a large bowl. For extra crispness, pat the slices dry with paper towels to remove surface moisture.

Make batter and dredge mix

In a medium bowl whisk together the all-purpose flour, cornstarch, fine salt, black pepper, paprika (if using), and baking powder. In a separate bowl whisk the buttermilk and egg until combined. Reserve 2 tablespoons of the dry mixture in a shallow dish for a final dusting.

Coat onions

Toss the onion slices in the buttermilk-egg mixture so they are evenly moistened. Working in batches, transfer moistened onions to the dry flour mixture and toss to coat thoroughly. Shake off excess flour. For a thicker, crispier coating, dip the coated onions back into the buttermilk briefly and then re-dredge in the dry mixture. After dredging, spread the coated onions on a wire rack and let rest 5 minutes; this helps the coating adhere.

Heat oil

Pour neutral frying oil into a heavy-bottomed pot or deep skillet to a depth of about 2–3 inches (5–7 cm). Heat the oil over medium-high heat to 175–180°C (350–360°F). If you don’t have a thermometer, test by dropping a small pinch of the flour mix into the oil — it should sizzle and rise immediately without burning.

Fry onions

Fry the coated onions in small batches (so they don’t clump or drop the oil temperature) for 2–3 minutes per batch, stirring gently with a slotted spoon or spider so they separate and brown evenly. Remove when golden-brown and crisp. Drain on a wire rack set over a baking sheet or on paper towels. Immediately season lightly with additional fine salt while still hot.

Finish and serve

If you want extra-crispy results, place the drained onions in a 120°C (250°F) oven for 8–10 minutes to dry further, then let cool to room temperature — they will crisp more as they cool. Transfer to a bowl and sprinkle with optional chopped parsley if desired. Use as a topping for salads, green bean casserole, burgers, mashed potatoes, or eat as a snack.

Storage

Once completely cool, store leftover crispy onions in an airtight container at room temperature for up to 3 days. Re-crisp in a 150°C (300°F) oven for 5–8 minutes if they soften.