Preheat the oven to 350°F. Generously butter a 9-inch x 13-inch casserole dish.
Place a large skillet over medium-high heat. Cook the sausage, breaking it up as you go until it is no longer pink. Set aside in a very large bowl.
Melt butter in skillet over medium heat. Add the onion and celery along with salt and pepper. Cook until the vegetables become tender, about 10 minutes. Cool slightly. Add the celery and butter mixture to the bowl with the sausage.
Pour the box of stuffing, apples, and cranberries over the sausage, onion, and celery in the very large bowl. In a medium bowl, whisk together the chicken stock, egg, parsley, and sage. Drizzle the chicken stock-egg mixture over the stuffing, season with salt and pepper, and mix until combined. Continue adding stock until desired consistency is reached.
Pour the stuffing into the buttered casserole dish and cover tightly with foil.
Bake 20 minutes with the foil on, then remove the foil and cook for 20 minutes more.
To make ahead, follow steps 2-5. The stuffing can be made 2 days prior to baking and serving. For best results, assemble a day before baking. Make sure to cover tightly before storing in the fridge.