Whisk together flour, cornstarch, spices, club soda until smooth. Refrigerate 10 minutes.
Meanwhile spread Panko in a shallow bowl and cut onions into thick 3/4″ rings.
Heat oil to 350ºF
Stir batter once more, thinning with additional water if necessary (should be a little thicker than pancake batter) and use a fork to dip onion rings into batter.