Cut onions into rings. 1/4" works well, but you can go slightly thicker.
Combine flour, cornstarch, potato flakes, cayenne, and salt in a pan.
Add club soda and mix until dissolved. Don't worry about over-mixing, but don't spend forever trying to making it completely smooth. Prep a second pan with panko breadcrumbs with one fork for each pan and heat about ~1" of oil in a pot to 350F.
Dip a ring into the batter and make sure rings are completely covered inside and out, the drop into the pan of breadcrumbs.
Now use the dry fork to cover the ring completely, and then repeat with the remaining rings.
Fry the rings in 350F, working in batches. About 2-3 minutes per batch until golden brown.
Remove from the oil to a rack or a bed of paper towels and season with some salt while hot.