Peel the onions and slice into 1/4- to 3/8-inch (6–9 mm) rings. Gently separate rings from each other. If some inner rings are very small, reserve for another use or stack two together to make a larger ring.

Place the rings in a bowl of cold water for 10 minutes to remove some of the sharpness; drain and pat dry thoroughly on paper towels. Dry rings help batter and breading adhere and reduce oil splatter.

Batter

Make dry mix

In a large bowl, whisk together 1.5 cups all-purpose flour, 1/4 cup cornstarch, 1 tsp baking powder, 1 tsp salt, 1/2 tsp black pepper, and 1/2 tsp paprika until evenly combined.

Add wet

In a separate small bowl, beat 1 egg lightly. Add the egg to the dry mix, then slowly whisk in 1 cup cold sparkling water until a smooth batter forms. The batter should be thick enough to coat the back of a spoon but still pourable; adjust with a tablespoon more flour if too thin, or a tablespoon more water if too thick. Keep the batter cold by placing the bowl over another bowl with ice if not using immediately.

Breading

Set up dredging stations

Place 1/2 cup flour in a shallow bowl for dredging. In a second shallow bowl pour the prepared batter. In a third shallow bowl combine 1.5 cups breadcrumbs (or panko) with 1/4 tsp salt and 1/4 tsp black pepper and mix.

Working in small batches to avoid crowding, dredge each dry onion ring in the 1/2 cup flour, shake off excess, dip into the batter ensuring an even coat, then press into the breadcrumbs to fully coat. Place breaded rings on a wire rack set over a baking sheet to rest for 5–10 minutes; this helps the coating adhere and prevents it from falling off in the oil.

Frying & Finishing

Heat oil

Pour about 3–4 inches (enough for deep frying) of oil into a heavy-bottomed pot or deep fryer (approx. 1 liter) and heat to 350°F (175°C). Use a thermometer to maintain temperature; if you don't have one, test by dropping a small amount of batter — it should sizzle and rise to the surface steadily.

Fry the rings in batches (do not overcrowd) for 2.5–3.5 minutes per batch, or until golden brown and crisp. Turn rings as needed with a slotted spoon or spider to ensure even browning. Maintain oil temperature between 340–360°F (170–180°C); if temperature falls, rings will absorb more oil and become greasy.

Remove fried onion rings with a slotted spoon and transfer to a paper towel–lined baking sheet or wire rack. Immediately sprinkle with 1 tsp finishing salt (or to taste) while still hot. Repeat with remaining rings.

Let rest 1–2 minutes, then serve hot with your choice of dipping sauces (ranch, ketchup, spicy aioli) or as a side.