In a large bowl, combine the cornmeal, flour, sugar, baking soda, baking powder and salt. In a separate bowl, add the buttermilk, onion, and egg, whisking well. Add the wet ingredients to the dry and mix until just combined.
1 cup ground cornmeal, ⅓ cup all-purpose flour, 1 tablespoon granulated sugar, 1 teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon kosher salt, ¾ cup buttermilk, ¼ cup grated sweet onion, 1 large egg