Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish. Set it aside.
In a large mixing bowl, combine the chicken, 1 ½ cups of cheese, sour cream, milk, onions, and your desired seasonings. Stir until well mixed, then transfer the mixture to the prepared baking dish, spreading it out evenly.
In a separate bowl, prepare the cornbread batter by whisking together the cornbread mix, corn, eggs, and melted butter. Stir gently until just combined; it’s fine if the batter has a few lumps. Avoid overmixing.
Pour the cornbread batter over the chicken mixture in the baking dish, using a spatula to spread it evenly and cover the entire surface.
Sprinkle the remaining cheese over the top for a perfectly cheesy finish.
Bake the casserole in the preheated oven for 35-45 minutes, or until the cornbread topping is golden brown and a toothpick inserted into the center comes out clean.
Once baked, remove the casserole from the oven and let it cool for a few minutes before serving. Slice into squares and enjoy warm.