Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish.
Cook Sausage: In a large skillet over medium heat, cook the breakfast sausage, breaking it up with a spatula, until browned and fully cooked. Remove the sausage with a slotted spoon and set aside.
Sauté Aromatics: In the same skillet, melt the 6 tablespoons of butter over medium high heat. Add the chopped onions and celery (if using). Cook for 15-20 minutes, stirring occasionally, until the onions are softened and golden brown.
Add Apples: Add the diced apples to the skillet and cook for 5 minutes more, until slightly softened. Remove from heat.
Combine Base: In a very large bowl, combine the bread cubes, cooked sausage, and the onion apple mixture. Add the sage, parsley, salt, and pepper. Toss to combine.
Add Liquid: In a small bowl, whisk together the chicken broth and eggs. Pour this mixture over the bread mixture. Add the dried cranberries. Gently toss until all the bread cubes are evenly moistened.
Transfer to Dish: Transfer the stuffing mixture to the prepared baking dish. If desired, drizzle the top with the 2 tablespoons of melted butter.
Bake: Cover the dish with aluminum foil. Bake for 30 minutes. Remove the foil and bake for an additional 10-14 minutes, until the top is golden brown and crispy and the stuffing is heated through.
Rest and Serve: Let the stuffing rest for 10 minutes before serving.