Preheat frying oil to 350°F (175°C)
Slice onions into ½-inch rings and separate. Lightly dust with gluten-free flour
Whisk together all dry batter ingredients. Add about ⅓ of the ice-cold water to form a thick paste, then slowly whisk in the remaining water until smooth and lump-free
Dip floured onion rings into the batter, letting excess drip off. Fry in batches for 4–5 minutes, or until golden and crispy. Drain on a rack
In a saucepan, combine lime juice, lemon juice, water, sugar, vinegar, fish sauce, kosher salt, and citrus zests. Bring to a simmer, then stir in the cornstarch slurry. Cook until thickened and glossy
Serve onion rings hot with the Sprite sauce on the side. Enjoy immediately
