2 large yellow onions, sliced crosswise into 1/2-inch thick rounds and separated into individual rings
2 cups buttermilk
1 cup all-purpose flour
1/2 cup finely ground cornmeal
1 tsp salt
3 cups vegetable oil,as needed
Ketchup, for serving (optional)
Put the onions in a large bowl and add enough buttermilk to cover. Cover the bowl and refrigerate for at least 1 hour and up to 12 hours.
Line a plate with a few layers of paper towels.
Pour 1 inch of oil into heavy-duty pot . Heat over medium heat until the thermometer registers 350°F. Working in batches of 4 or 5, remove the rings from the buttermilk, letting any excess drip off, then dredge in the flour mixture, knocking off the excess.
Carefully lower the rings into the oil and fry until lightly browned on one side, about 2 minutes. Using tongs, gently turn the rings over and continue to fry, adjusting the heat as necessary to maintain a steady temperature,until golden-brown all over, about 2 minutes. Transfer to the paper towels to drain. Sprinkle generously with kosher salt. Serve with ketchup,if you like.