Slice onion very thinly. I use a mandolin for this task.
Place in a baking dish and cover with buttermilk and soak for 1/2 an hour.
Combine dry ingredients and set aside.
Heat oil to 375 degrees. Use a candy/ oil thermometer for accuracy.
Put a handful of onions into the flour mixture, tap to shake off excess. Gently place into hot oil. Fry for a few minutes and remove as soon as they are golden brown. Remove to a paper towel