In a large bowl, whisk together the buttermilk, 1 ½ teaspoons kosher salt, and 1 teaspoon black pepper.

Peel the onions and slice into ½- to ¾-inch thick rounds. Separate into individual rings. Add the onion rings to the buttermilk mixture and toss to coat. Let soak for at least 15 minutes, or up to 4 hours in the refrigerator.

In a separate bowl, whisk together the flour, cornmeal, remaining 1 teaspoon kosher salt, and remaining ½ teaspoon black pepper.

Pour the vegetable oil into a Dutch oven or large heavy-bottomed pot. Heat over medium-high heat until the oil reaches 350°F (175°C). Use a clip-on thermometer to monitor the temperature.

Working in small batches of 4 to 5 rings, remove the onion rings from the buttermilk and let the excess drip off for a few seconds. Toss in the flour-cornmeal mixture until fully coated, shaking off any excess. Carefully place the coated rings into the hot oil.

Fry for about 2 minutes per batch, turning once halfway through, until golden brown. Do not overcrowd the pot.

Transfer the fried onion rings to a wire rack set over a baking sheet. Sprinkle lightly with salt while still hot. Repeat with remaining rings, letting the oil return to 350°F between batches. Serve immediately.