Step 1: Prepare the Onions

1. Slice and separate: Slice the onions into 1/2-inch rings and separate them. Pat dry with a paper towel to remove excess moisture.

Step 2: Make the Batter

1. Mix the dry ingredients: In a large bowl, whisk together the flour, cornstarch, baking powder, salt, pepper, and paprika.
2. Add wet ingredients: In another bowl, whisk the buttermilk, egg, and sparkling water until combined. Gradually add the wet mixture to the dry ingredients, whisking until smooth.

Step 3: Heat the Oil

1. Prepare for frying: Pour vegetable oil into a heavy-bottomed pot or deep fryer, filling it about 2–3 inches deep. Heat the oil to 350°F (175°C).

Step 4: Coat and Fry the Onion Rings

1. Dip the rings: Dip each onion ring into the batter, letting any excess drip off.
2. Fry in batches: Carefully place a few onion rings in the hot oil, frying for 2–3 minutes per side or until golden brown. Avoid overcrowding the pan.
3. Drain and season: Remove the onion rings with a slotted spoon and place them on a paper towel-lined plate. Sprinkle with a pinch of salt while hot.

Step 5: Serve

1. Plate and enjoy: Serve the onion rings immediately with your favorite dipping sauce, such as ketchup, aioli, or ranch dressing.