01 - Peel onions and slice into ½-inch thick rings. Separate rings and set aside.
02 - In a large bowl, whisk together flour, cornstarch, baking powder, salt, black pepper, and paprika.
03 - Gradually whisk in cold sparkling water until batter is smooth and thick enough to coat the back of a spoon.
04 - Heat vegetable oil in a deep pot or fryer to 350°F (175°C).
05 - Dip each onion ring into batter, let excess drip off, then coat thoroughly with panko breadcrumbs.
06 - Fry rings in batches for 2 to 3 minutes, turning once until golden and crisp.
07 - Remove rings with a slotted spoon, drain on paper towels, and serve hot with dipping sauce.