Separate rings and place in a mixing bowl and cover with buttermilk. Cover bowl and refrigerate 2-24 hours.
Pour oil to a 2 inch depth in a deep pan and heat to 360.
Stir flour with cornmeal, salt and cayenne.
Dredge onion rings in flour mix and lay out on to a baking sheet.
Fry in batches and drain on paper towels. Salt immediately after frying. Keep warm in a 200 oven until serving.
Serves 3-4 as a garnish, double the recipe to serve as a side dish.