Add dry ingredients to bowl: Combine first 5 ingredients in a bowl. Make a well in the center of the mixture.
Combine wet and dry ingredients: Stir together egg and buttermilk. Add mixture to dry ingredients. Stir until moistened. Stir in onion.
Fry hush puppies: Pour oil into 10-inch cast iron skillet, and heat to 375°F. Drop batter by tablespoonfuls into oil, and fry in batches 3 minutes on each side or until golden. Drain on paper towels. Serve immediately.
