Preheat oven to 425°F (220°C).
Place a 9-inch cast iron skillet in the oven while it preheats.
In a large bowl, whisk together the cornmeal, flour, baking powder, baking soda, salt, and sugar (if using).
In a separate medium bowl, whisk the eggs, buttermilk, and melted butter until smooth.
Pour the wet ingredients into the dry ingredients and stir just until combined. Avoid overmixing.
Gently fold in the finely chopped Vidalia onion and shredded cheddar cheese (if using).
Carefully remove the hot skillet from the preheated oven. Add the vegetable oil or bacon grease to the skillet, ensuring it coats the bottom.
Pour the prepared batter into the hot skillet and spread it evenly.
Bake for 20–25 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Allow the cornbread to cool in the skillet for a few minutes before slicing and serving.
