After slicing onions put into a shallow bowl and cover with buttermilk, let sit for hour in fridge.
Take a cup of buttermilk from the soaking mixture and pour out the rest.
Add flour to a bowl with corn starch, this lightens up the gluten. Add cayenne pepper, black pepper, and garlic powder to flour. Do not salt batter as it makes the onions sweat and batter will fall off.
Dredge onion rings into flour and set on cookie rack, now add buttermilk to flour and 2 tablespoon of instant potatoes and mix. Let mixture sit for 10 min or til the batter is nice and thick.
Dip onion rings in batter and then into Panko bread crumbs, this makes them so incredibly crunchy you won't believe it. Put onion rings on a cooling rack to dry for at least 10 mins. Drying keeps the batter/Panko on better when cooked and less greasy.
Deep fry after oil has reached 350 degrees, flipping over after couple mins until golden brown. Remove to paper towels or cookie rack to drain. Salt right after they come out of oil.