Heat oven to 375°F.
In 10-inch cast iron skillet, heat 2 tablespoons oil or butter over medium heat. Gently place onion slices in skillet close together in a single layer. If all rings do not fit, remove outside rings until small enough to fit. Cook 6-8 minutes without stirring, until onions soften but don’t fall apart. Remove from heat, leaving onions in skillet.
Whisk together milk, butter, and eggs. Add cornbread mix and stir until blended.
Spoon batter into skillet on top of onions, being careful to keep onions evenly arranged in skillet.
Bake 20-22 minutes or until light golden brown.
Run knife around edges of cornbread to loosen. Let cornbread cool in skillet for 10 minutes.
Place plate or sheet pan upside down on top of skillet and carefully flip so cornbread drops onto plate/pan.