In a bowl, mix together the onion, salt, baking soda, flour and cornmeal.
Heat oil in a saucepan over medium-high heat until it reaches 375 degrees F (190 degrees C).
In another bowl, mix together the buttermilk and egg.
Add the wet ingredients to the dry ingredients and mix together.
Drop tablespoons of the batter into the hot oil and fry until golden brown, about 2 minutes per side.
Remove the hush puppies from the oil and drain on paper towels. Serve immediately.