In a large bowl, whisk together cornmeal, flour, baking soda, baking powder and salt. Stir in onions. Combine egg yolk and buttermilk; stir into cornmeal mixture until combined. In the bowl of an electric mixer fitted with the paddle attachment, beat egg whites until stiff. Gently fold into cornmeal mixture. Fill a large deep skillet with 2 to 3 inches oil. Heat oil until it reaches 375 degrees on a deep-fry thermometer. Working in batches, drop batter 1 t at a time into the oil and cook, turning once until the hush puppies rise to the surface and are golden brown. 2 to 3 minutes. Transfer to a paper-towel-lined plate. Serve immediately.