Preheat oven to 400°. Coat bottom and sides of a 10-inch cast-iron skillet with 1 Tbsp. oil; heat in oven 10 minutes.
Meanwhile, stir together cornmeal mix, next 3 ingredients, and remaining 1 Tbsp. oil. Pour batter into hot skillet.
Bake at 400° for 30 minutes or until golden. Remove from oven to a wire rack; cool 15 minutes. Crumble cornbread into a large bowl. Stir 5 cups broth into crumbled cornbread until moistened, adding more broth, 1 Tbsp. at a time, if necessary. (Mixture should resemble wet sand.)
Melt 1/4 cup butter in a large skillet over medium-high heat; add onion and celery, and sauté 8 to 10 minutes or until tender. Add onion mixture to cornbread mixture.
Microwave remaining 1/2 cup butter in a small microwave-safe bowl at HIGH 1 minute or until melted. Stir melted butter, eggs, and remaining ingredients into cornbread mixture; spoon into a lightly greased 13- x 9-inch baking dish.
Bake at 400° for 50 minutes to 1 hour or until golden brown. Serve immediately.