Preheat the oven to 375 degrees.
Crumble the previously made cornbread into a large mixing bowl.
Melt 2 tablespoons of butter over medium-high heat in a skillet. Once melted, add the onions and celery to the skillet, and cook for 1-2 minutes.
Pour ½ c. of bone broth into the skillet, and saute the vegetables until almost all of the liquid has disappeared. Remove the onions and celery from the skillet and set aside to cool slightly.
Place another tablespoon of butter into the same skillet, and toss in ⅓ c. of sage leaves. Let them cook for 20-30 seconds until crispy, and remove.
Add the cooked sage leaves and the beaten egg to the cornbread in the mixing bowl. Toss to combine.
Mix in the cooked vegetables and melted butter.
Slowly pour in 1 cup of bone broth over the cornbread mixture, a little at a time. Stir as you pour to make sure the broth gets incorporated into the entire mixture.
Add in 1 teaspoon of poultry seasoning and salt and pepper to taste.
Place the cornbread mixture into a well greased casserole dish and bake for 20 minutes until the top is browned.