Mix seasonings, cornstarch, and flour together in a bowl.Pour buttermilk into a separate bowl and add bread crumbs to a third bowl.
Heat oil in a skillet or Dutch oven to 375 degrees. Line a platter with paper towels or place a rack on top of a baking sheet to allow oil to drip off. Set aside.
Dip each ring into cold buttermilk, then dip in flour mixture.Dip once again in buttermilk and finally into the breadcrumbs.
Place dipped rings onto a plate until ready to fry.
Fry onion rings for 2-3 minutes, flipping halfway through. Onion rings are done when they are golden brown.
Place fried onion rings on a rack to allow excess oil to drip off. Sprinkle hot onion rings with additional salt, if desired.
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Notes
What kind of oil should I use for frying?Vegetable oil is a great all-around oil for deep frying and frying, as it has a high smoke point. Peanut oil is another great option. It's best to save olive oil and coconut oil for pan frying or sautéing, as it has a lower smoke point.
Do I need to dip the onion rings in bread crumbs?If you'd like to omit the breadcrumbs altogether and dip the onion rings into the flour mixture twice, you can do that as well. That would give you more of a crispy batter onion rings vs. a crispy crumb onion ring.
How do I reheat onion rings?If you have leftovers, you can easily reheat the onion rings on day 2 in the oven or air fryer.