Slice the onions into ½-inch thick rings and separate them. Pat the rings completely dry with a towel.

In a large bowl, whisk together the flour, cornstarch, paprika, garlic powder, and salt. In a small bowl, beat the egg with the sparkling water, then pour into the dry ingredients. Whisk until a smooth batter forms.

Dip each dry onion ring into the batter, letting the excess drip off. Place them on a parchment-lined baking sheet as you go.

For frying: heat oil to 350°F (175°C) and fry in batches for 2 to 3 minutes, turning once, until deep golden. For baking: preheat oven to 425°F (220°C), arrange rings on a wire rack set over a baking sheet, spray with cooking spray, and bake for 18 to 22 minutes, flipping halfway.

Drain fried rings on a wire rack. Sprinkle immediately with flaky salt and serve while hot.