Preheat oven to 350 degrees and lightly grease a casserole dish.: You will notice a warm, anticipatory stillness in the kitchen as the oven reaches 350 degrees , and preheating ensures even baking from the moment the dish goes in. The greased casserole prevents sticking and helps the edges brown evenly, producing small golden pockets along the rim. A common mistake is not preheating, which can lengthen bake time and yield uneven texture, so give the oven time to come to temperature.
Add ground sausage to a pan and cook until no longer pink, breaking it apart with a wooden spoon as it cooks. Remove from pan and drain on paper towel-lined plate.: As the sausage hits the hot pan you will hear a steady sizzle and smell savory meat juices. Browning develops complex flavors through the Maillard reaction, giving the filling depth. Drain excess fat on paper towels to avoid greasiness, and ensure the crumbles are small for even distribution. Avoid crowded pans, which steam the sausage rather than brown it; cook in batches if needed.
Wipe pan clean and heat 4 tablespoons of butter. Add onion and celery. Cook until soft, about 6 to 8 minutes. Return sausage to skillet.: When you melt the butter , the pan will shine and the aroma becomes rich and nutty, a perfect base for the chopped onion and celery . As they soften, their scent sweetens and the texture goes tender, signaling readiness. Returning the browned sausage lets the flavors marry; the vegetables will pick up meat drippings for cohesion. Burning is the common error here, so keep the heat moderate and stir regularly for even softening.
Add sage and poultry seasoning to skillet and give it a good stir. Remove from heat and transfer mixture to a large bowl.: Freshly added dried sage and poultry seasoning will bloom in the warm fat, releasing herbal perfume and rounding out the savory notes. Stirring distributes the spices so no single bite is too intense. Moving the pan off the heat prevents the herbs from becoming bitter or overly toasted, which can happen with prolonged high heat.
Add crumbled cornbread and stuffing mix to bowl along with all other ingredients except slivered butter. Stir well and pour into prepared casserole dish.: The moment you add the cornbread and stuffing mix, you will begin to feel the mixture take shape; it should be moist but not soggy. The condensed soup, beaten egg , salt, pepper, and measured chicken broth bind the crumbs and create a tender interior. Mix until components are evenly coated, pressing lightly to check consistency. If it seems dry, gradually add more chicken broth ; if it is too wet, a few more crumbs will help. Overmixing can make the texture gluey, so stop once homogenous.
Scatter slivered butter on top. Place in oven and bake for 30 to 35 minutes.: As the casserole bakes, the butter melts and the surface begins to brown, releasing buttery aroma while the interior heats through and sets. You will hear gentle bubbling and see edges turn golden, a clear visual cue it is done. Allow it to rest for a few minutes after removal to set up before serving. A trap is removing it too early, which leaves the center underdone; follow the time and check for a lightly crisp top and warm, steaming center.