Preheat the oven to 375°F.
Place 2 tbsp butter in a skillet over medium-high heat.
Allow the butter to melt completely.
Add the chopped onion and chopped celery to the skillet.
Sauté until the vegetables are softened.
In a large mixing bowl, combine the cubed cornbread or dried bread, shredded chicken, sautéed onion and celery, dried sage, poultry seasoning, cream of onion soup, and chicken broth.
Stir until all ingredients are evenly combined.
Season with salt and black pepper to taste.
Grease a 9 x 13-inch baking dish.
Transfer the chicken and dressing mixture to the prepared baking dish.
Spread the mixture evenly in the dish.
Bake for 45 minutes, or until the casserole is hot throughout and lightly golden on top.
While the casserole bakes, prepare the gravy.
Melt 1/4 c unsalted butter in a skillet over medium heat.
Add the flour to the melted butter.
Whisk until the flour and butter are fully combined.
Continue cooking, whisking frequently, until the mixture begins to brown slightly.
Slowly whisk in the chicken broth.
Season with salt and black pepper to taste.
Continue simmering until the gravy begins to thicken.
Stir in the heavy cream.
Simmer for an additional 5 to 10 minutes, or until the gravy is smooth and creamy.
Remove the casserole from the oven.
Pour the gravy over the top of the casserole, or serve it on the side.
Garnish with parsley or chopped green onion, if desired.
Serve warm.