Directions
In a large shallow bowl, toss onions with buttermilk. In another large bowl, whisk together flour, cayenne, 1/2 teaspoon salt, and 1/4 teaspoon ground pepper. Sprinkle 1/2 cup flour mixture evenly on a rimmed baking sheet; set aside.
In a 5-quart heavy-bottom saucepan, heat oil until a deep-fry thermometer registers between 325 degrees and 350 degrees. Lift rings out of the buttermilk (letting excess drip off); toss with remaining flour mixture to coat. Place on floured sheet.
Working in small batches, carefully place coated rings (shaking off excess flour) into hot oil. Cook until golden, gently turning with tongs, 2 to 3 minutes. Transfer rings to a paper-towel-lined baking sheet. Repeat with remaining rings. Serve on Arugula Salad, if desired.