Separate cut onions into individual rings.
In a bowl, combine flour, cornstarch, instant mashed potatoes and cayenne powder.
Add club soda and stir to combine. Set aside for about 1 minute as it slightly thickens.
In a shallow dish, place panko crumbs.
Dip individual pieces of onions into the batter to coat and then roll in panko crumbs.
In a pan over medium heat, heat about 2 inches deep of oil to 350 F.
Add panko-coated rings in batches as needed and deep-fry, turning once or twice, for about 2 to 3 minutes or until golden and crisp.
Remove from pan, drain on a wire rack and season with salt to taste.