Preheat the oven to 375˚F. Grease the bottom and sides of a 9x13 inch casserole dish with a small pad of unsalted butter. Set it aside for later.
Heat a large pan with sides over medium high heat. Add the sweet Italian sausage to the pan. Cook and crumble the sausage until it is completely cooked through. Transfer the sausage to a bowl and set it aside for later.
Pour out any excess oil from the sausage that is left in the pan. Heat the pan over medium heat and add in the butter. Allow the butter to melt then add in the onion, shallot, and garlic. Sweat the vegetables until the onions turns translucent. Mix in the celery and mushrooms and continue to cook the vegetables until the celery is tender.
Remove the pan from heat then add in the sage, rosemary, thyme, parsley, cooked sausage, salt, and pepper. Mix until the ingredients are combined.
Whisk the eggs and chicken stock together in a medium sized bowl. Add the dried bread and the chicken stock to the pan. Gently fold everything together until the cubes of bread become soggy.
Pour the stuffing into the prepared casserole dish and spread it out into an even layer. Cover the top of the dish with aluminum foil and bake it in the oven for about 30 minutes. Then remove the foil covering and increase the temperature to 400˚F. Bake the stuffing in the oven uncovered for about 20 minutes. The top layer should become slightly crispy and golden brown.
Remove the stuffing from the oven and allow it to cool for a few minutes before serving. Enjoy!