In a medium bowl, combine the dry ingredients (flour, cornstarch, cayenne, S&P)
Slowly pour in the cup of cold water, whisk, and mix well
In another large bowl, pour the panko bread crumbs
Slice the onion into ¼ inch slices and separate the rings
Dip rings three times in the sauce, then into panko and onto a large plate or baking sheet
Heat the vegetable oil in a cast iron skillet to 350 degrees or medium heat (as you add rings, the oil should bubble)
Work in batches - dropping rings into the oil, turning once or twice until golden. Approx. 1-2 minutes
Remove and place on a paper towel or rack until finished
Stack on a plate, salt lightly, and serve right away with catsup or dipping sauce