Cut onion into 1/2-inch slices and separate into rings. In a large mixing bowl, combine the flour, cornmeal, baking powder, salt, and baking soda. Combine egg and buttermilk, then stir into dry ingredients just until moistened. In deep fat fryer, heat 1 inch of oil to 375 degrees. Dip onion rings into batter. Fry a few at a time for 1 to 1 1/2 minutes or until golden brown. Drain on paper towels (keep warm in a 300-degree oven).