Preheat oven to 400 degrees.
In a medium bowl, stir together all ingredients for cornbread: self-rising cornmeal, self-rising flour, buttermilk, eggs, and vegetable oil. Mix until well combined.
Pour the batter into a lightly greased 9-inch cast iron pan or a 9-inch baking pan.
Bake for 20 to 25 minutes or until the top is golden brown.
In a large pan, heat the butter over medium heat. Add celery and onion and cook until soft.
Add dried sage, poultry seasoning, salt, and pepper to onion mixture.
In a large bowl combine crumbled cornbread and toast.
Whisk together milk and eggs, add to bowl. Stir in chicken broth.
Fold in the onion mixture.
Transfer to a greased baking dish and scatter butter on top.
Bake at 350 degrees for 30 minutes, or until it turns light brown on top.