01 - Peel the onions and slice them into very thin rings (about 1/8 inch thick). Carefully separate the rings.
02 - In a large bowl, combine the flour, cornstarch, baking powder, garlic powder, paprika, salt, and black pepper.
03 - In a separate bowl, whisk together the cold sparkling water and egg.
04 - Add the wet mixture to the dry ingredients and whisk until just combined; do not overmix. The batter should be smooth and slightly thick.
05 - Heat oil in a deep fryer or large, heavy-bottomed pot to 350°F (175°C).
06 - Working in batches, dip the onion rings into the batter, allowing excess to drip off, then carefully place them into the hot oil.
07 - Fry for 2-3 minutes per batch, turning once, until golden brown and crisp.
08 - Remove with a slotted spoon and drain on paper towels. Repeat with remaining onion rings. Serve immediately, optionally with your favorite dipping sauce.