Start by preheating your oven to 350°F. While the oven heats, prepare the stuffing according to the instructions on the box. The stuffing creates the signature crispy top and adds classic holiday flavors.
Melt butter in a large skillet over medium heat. Add the celery, onion, and minced garlic, cooking until the vegetables soften and the aroma fills the room. This step builds the flavor base of the dish.
Mix cornstarch and water in a bowl until it forms a smooth liquid. Pour this into the skillet with the veggies. Add milk and chicken bouillon, stirring constantly. Bring the mixture to a gentle boil until it thickens into a creamy sauce. Remove from heat and stir in sour cream to make it even richer.
Grease your 9x13 baking dish, then layer the chopped rotisserie chicken at the bottom. Pour the prepared sauce evenly over the chicken layer. Finally, top the dish with the cooked stuffing, spreading it out to create a crunchy, golden layer.
Bake the casserole uncovered for 45 minutes to an hour, or until the top is golden and crispy. The stuffing forms a delightful crust while the interior stays creamy and savory.
Once the casserole is done, remove it from the oven and allow it to cool for 5 minutes before serving. This resting period helps the layers set slightly, making it easier to dish out.