The simple batter for these onion rings results in a ridiculously crunchy ring. I added some habanero powder to the batter as well as the panko breadcrumbs. Frequent flipping in the frying oil resulted in a ring with a more even crunchy coating.
Chop the top and bottom off the onion and remove the outer layer. Slice in to thirds and separate in to rings.
Combine salt, corn starch, flour and any additional seasoning, I used habanero powder, in a mixing bowl. Dredge onions in flour and set aside.
Whisk in 1 cup of any bubbling liquid like seltzer or tonic (or beer) until a batter forms. Small lumps are fine. Dredge onion rings in batter and set aside. If not frying immediately, rest in refrigerator.
Heat 4 cups frying oil in a large heavy pot or dutch oven to 375°F. Test one onion ring for proper temperature. Onion should begin frying rapidly. Fry for one minute per side flipping occasionally until dark golden brown.
Dust with a pinch of fine sea salt as they're removed from the hot oil.
Notes
I prepared these with some habanero chili powder in the batter and the panko bread crumbs. You can add any sort of additional spices you'd like such as black pepper, cayenne, nutmeg, or sumac.The dipping sauce shown is sriracha lime mayo. Mix equal parts sriracha and mayo and add 1 tsp lime juice.