Preheat your oven to 375° F. Grease a 9x13-inch baking dish and set aside. In a large bowl, mix together the cooked and diced or shredded chicken, 1 ½ cup cheese, sour cream, milk, onions, and seasonings.
Transfer the chicken mixture to the prepared baking dish. Spread it out evenly. In a mixing bowl, whisk together the cornbread mix, corn, eggs, and butter. In a separate bowl, whisk together the milk, melted butter, and egg.
Stir until just combined. Be careful not to over-mix; a few lumps are okay. Pour the cornbread batter over the chicken mixture in the baking dish. Use a spatula to spread it evenly, covering the entire surface. Sprinkle the remaining cheese over the top of the casserole.
Place the casserole in the preheated oven and bake for about 40 minutes, or until the cornbread topping is golden brown and a toothpick inserted into the center comes out mostly clean with moist crumbs attached.
Remove from the oven and let it cool for a few minutes before serving. Cut into squares and serve warm.