Preheat deep fryer to 375°F, or if using a large Dutch oven pot, fill 2-3” deep with oil, use a deep-fry thermometer and preheat to 375°F. 

Place 1 cup of flour in a bowl, add the sliced, separated onion rings and toss to dust in flour. In another large bowl mix together the corn starch, salt, baking powder, baking soda and cayenne pepper. Add in the club soda and whisk until smooth.

Working in batches, dip your dusted onion rings into the batter, allow excess to drip off before carefully lowering into the hot oil. Working in batches, deep-fry the rings for 2 to 3 minutes, turning over halfway, until golden brown. Remove to paper towels to drain. Season with salt if desired and serve immediately.