Place oil in a deep skillet until it comes abut 1 inch up the side of the pan. Heat oil to 360 F. Mix buttermilk, egg, and hot sauce in a medium bowl. Mix flour, cayenne, 1 teaspoon salt and pepper in a brown bag. Pat onion slices dry and dip in the buttermilk mixture to coat. Transfer slices to bag and toss to coat. Deep fry onions in batches until golden, about 2 minutes. Transfer to a paper towel lined plate to drain. sprinkle immediately with salt and serve.