Preheat the oven to 400℉. Spray a 9x9-inch baking dish with nonstick cooking spray.
In a large mixing bowl, mix the shredded chicken with 1 cup of the bbq sauce, corn, diced green chiles, 1/2 cup of the shredded cheddar cheese, and red onion.
2 cups cooked, shredded chicken, 1½ cups BBQ sauce, 1 can (15 oz) corn, 1 can (4 oz) diced green chiles, 1/2 cup finely diced red onion, 1 cup shredded cheddar cheese
Transfer the chicken mixture to the prepared baking dish and spread it out evenly.
In a separate medium bowl, mix together the Jiffy cornbread mix, whole milk, and large egg until combined. Spread the cornbread batter over the chicken mixture in the baking dish.
1 box (8.5 oz) Jiffy cornbread mix, 1/2 cup whole milk, 1 large egg
Drizzle the remaining 1/2 cup bbq sauce over the cornbread batter and use a knife to gently swirl it into the batter.
Sprinkle the remaining 1/2 cup shredded cheddar cheese on top of the bbq swirled cornbread batter.
Bake for 25-30 minutes, or until the cornbread is golden brown and a toothpick inserted into the center comes out clean.
Remove the casserole from the oven and let it cool for 5-10 minutes before serving it warm.