1. Preheat the oven to 400°. Melt the butter in a 10-inch cast iron skillet over medium heat; cook the onion and celery, stirring occasionally, until tender, about 10 minutes. Remove the vegetable mixture to a large bowl.
2. Pour the oil in the same skillet; place in the oven to heat for about 5 minutes.
3. Add the cornbread mix, chicken, corn, poultry seasoning, milk, and eggs to the vegetable mixture; blend well. Carefully remove the skillet from the oven; pour the batter into the hot skillet. Bake until golden brown, 30 to 35 minutes. Cut into wedges and serve.