Preheat your oven to 375°F and thoroughly grease a 9×13-inch baking dish with non-stick cooking spray or butter. Melt the 2 tablespoons of unsalted butter in a medium skillet over medium heat, drop in the chopped onion and celery, and sauté for 5 to 6 minutes while stirring frequently until the vegetables soften and become fragrant.

In a large mixing bowl, combine the crumbled cornbread, shredded cooked chicken, and the sautéed onion and celery mixture.

Add the condensed cream of onion soup, poultry seasoning, dried sage, and 1 ½ cups of chicken broth into the bowl. Season with salt and black pepper to taste, then fold the ingredients together gently until the liquid fully hydrates the cornbread crumb, ensuring a moist but uniform matrix.

Spread the dressing and chicken mixture evenly into your prepared 9×13-inch baking dish, smoothing the top surface into a level sheet with a spatula. Place uncovered on the middle oven rack and bake at 375°F for exactly 45 minutes until the core is hot and the edges set cleanly.

While the casserole bakes, prepare the gravy baseline. Melt ¼ cup of unsalted butter in a small saucepan over medium heat, sprinkle in the ¼ cup of all-purpose flour, and whisk continuously for 1 to 2 minutes to cook out the raw starch flavor and create a smooth, golden roux paste.

Slowly stream the 2 cups of chicken broth into the roux while whisking rapidly to prevent lumps from forming. Bring the liquid to a gentle simmer, stirring constantly for 3 to 4 minutes until it thickens into a stable, velvety gravy consistency, then turn off the burner.

Whisk the ¼ cup of heavy whipping cream straight into the warm gravy, adjusting with salt and black pepper to taste. Remove the hot casserole from the oven, let it stand for 5 minutes to stabilize its structure, and slice into thick squares to serve alongside the warm pan cream gravy.