Cut onions ¼ inch thick.In a shallow container with a lid, mix together the buttermilk and eggs.Place the onions in the container and seal with the lid.Shake onions in the buttermilk.Heat the oil in an electric fryer or a large cast iron dutch oven to 360 degrees.Sift together the flour and baking powder.Dip the onion rings into the flour one at a time.After dipping the onion rings into the flour, place them on a flat baking soda without touching each other.When the fryer reaches 360 degrees, add the onion rings.Do NOT let them touch.Fry for 3 minutes.Remove from the fryer and place flat on a large baking sheet lined with paper towels.Season them immediately and generously with Cajun seasoning.