Preheat oven to 375 degrees F (190 degrees C). Grease an 8-inch square baking dish.
Melt the butter in a small skillet over medium heat. Add the onion and sauté until softened, about 5 minutes. Remove from heat and set aside.
In a medium bowl, combine the cornmeal, flour, baking powder, salt and baking soda. In a separate bowl, mix together the buttermilk, egg and melted butter. Add the wet ingredients to the dry ingredients and stir until just combined.
Fold in the sautéed onion and cheddar cheese. Pour the batter into the prepared baking dish.
Bake for 25 to 30 minutes, or until a toothpick inserted into the center comes out clean.
Let cool before serving.
Interesting Facts
Cornbread is a traditional Southern favorite that dates back to the early 1700s.
The addition of cheese and onions to cornbread is a relatively recent invention.