Combine first 5 ingredients in a bowl; make a well in center of mixture.
Stir together egg and buttermilk; add to dry ingredients, stirring just until moistened. Stir in onion.
Pour oil into a 10-inch cast-iron skillet, and heat to 375°. Drop batter by tablespoonfuls into oil, and fry in batches 3 minutes on each side or until golden. Drain on paper towels, and serve immediately.