1 large sweet onion, cut into 1/4-inch-thick rings
In a medium saucepan, heat 1 inch of vegetable oil to 350°. In a shallow bowl, whisk together the flour, fine cornmeal, cornstarch and the 1 teaspoon of salt. In another shallow bowl, whisk the buttermilk with the egg until thoroughly combined.
Working in batches, dredge the onion rings in the flour mixture. Dip the rings in the buttermilk mixture, allowing the excess to drip off. Fry the rings until golden brown and crisp, about 1 minute per batch. Drain on paper towels and season with salt. Serve hot.
The recipe can be made up to 3 hours ahead. Recrisp before serving.