Preheat the oven to 400˚F. Place the bacon drippings and vegetable oil in a 10-inch cast-iron skillet; heat in the oven for 4 minutes.
Whisk together the egg and buttermilk in a large mixing bowl. Stir in the corn and onion.
Whisk together the cornmeal mix and flour in a medium bowl; gently stir into the buttermilk mixture.
Remove the hot skillet from the oven; carefully pour half of the hot drippings into the batter. (The drippings will sizzle and sputter.) Whisk to combine. Pour half of the batter into the skillet.
Sprinkle half of the cheese evenly over the batter. Pour the remaining half of the batter over the cheese and top with the remaining cheese.
Bake at 400˚F for 30 to 35 minutes or until golden brown and the cornbread pulls away from sides of the skillet. Let cool 15 minutes before serving.