Cook the sausage and bacon separately in a large skillet until cooked & crisp.
Reserve 1 tablespoon of bacon grease in the pan and add the onions, celery, and mushrooms. Cook, stirring often, until tender, about 5 minutes.
Preheat the oven to 350 degrees. Coat a large baking dish with cooking spray.
Combine the seasoning packet (included with the stuffing mix) with the broth and butter in a large saucepan over medium heat. Bring to a low boil for 3-5 minutes.
Place croutons in a large bowl along with onion, celery, mushroom mixture, bacon, sausage, and chestnuts.
Combine the well-beaten egg with the chicken broth.
Pour the liquid over the mixture and toss lightly until evenly coated.
Spoon the filling into the baking dish.
Cover and bake for 45 minutes.
Remove the cover and bake for an additional 15 minutes to crisp the top.