- Preheat oven to 425 degrees. Pour oil into 10-inch cast-iron skillet and place in oven to heat.
- While skillet is heating, in a large bowl combine cornmeal, flour, baking powder, sugar, and salt. Add sour cream, egg, and buttermilk; whisk until smooth. Fold in onions.
- Remove skillet from oven and carefully pour hot oil into batter. Fold in oil with a wooden spoon and pour batter into hot skillet.
- Bake until cornbread is golden brown and knife inserted in center comes out clean, about 35−40 minutes.