Preheat oven to 400° F. Place an 8 to 10-inch cast-iron skillet in the oven.
Heat a small frying pan over a medium heat. Add a tablespoon of butter and melt. Add the onions and saute until softened, about 2 minutes. Saute for another 3 minutes, until the pancetta is very lightly browned. Turn off the heat and set aside.
In a small saucepan, bring 3/4 cups of water to a boil. Remove from heat and pour in 1/2 cup of the cornmeal. Stir rapidly until you have a warm mush.
In a medium bowl, blend the remaining corn meal, flour, brown sugar, salt, baking powder, and baking soda.
Add the buttermilk, sour cream, and eggs to the cornmeal mixture and mix well with a spatula. Fold in the pancetta and onions.
Once the oven has reached the desired temperature, remove the pan from the oven. Add the remaining 3 tablespoons of butter to the pan and swirl to melt and grease the pan. Pour any excess butter into the cornmeal mixture.
Fold the pancetta and onions into the cornmeal mixture and stir until just incorporated with a wooden spoon. Don’t overwork the batter or the cornbread will be tough.
Carefully add the batter to the hot pan and smooth out the top. Bake for 20 minutes or until golden brown (check it with a toothpick in the center, it should come out clean-baking time will vary depending on the size of the pan you used). Serve warm.