Preheat oven to 400 degrees.
Peel one large yellow onion. Cut off 4 thin slices. Separate slices into 10 to 12 rings (various sizes). Coarsely chop remaining half of onion into small pieces. Set aside.
In a medium sized bowl, combine yellow cornmeal mix, self-rising flour (sifted, spooned and leveled), sugar (if using), egg, and buttermilk (1 cup plus 3 tablespoons). Stir together until well combined. Batter will be pourable. Small lumps are okay.
Add coarsely chopped pieces of yellow onion into cornbread batter. Stir until well combined. Set aside.
THICK CORNBREAD: Use an 8" cast iron skilletTHIN CORNBREAD: Use a 9" cast iron skillet Pour 3 tablespoons vegetable oil into preferred cast iron skillet. Warm on top of stove over low-medium heat until oil sizzles. Avoid smoking hot. Turn off heat. Carefully arrange "onion rings" around bottom of skillet.
Test to see if oil is hot by dropping in a small dot of cornbread batter. If it quickly turns brown, pour in remaining cornbread batter.
Stir cornbread batter until smooth. Carefully pour into hot skillet.
Reduce oven to 375. Bake 20 to 25 minutes OR until knife inserted in center comes out clean and cornbread is golden brown. Watch carefully to prevent burning.
After baking, carefully flip upside down. The bottom crust covered with onion rings makes a pretty presentation.
Serve hot.