This recipe has been updated and may differ from what was originally published or broadcast.
Special equipment: cheesecloth
Preheat the oven to 350ºF.
Place the turkey on a flat surface and sprinkle inside and out with salt and pepper. Transfer the turkey to a large roasting pan and roast until an instant-read thermometer inserted into the thickest part of the thigh meat (without touching bone) registers 155 to 160ºF, 2 hours 55 minutes to 3 hours 15 minutes. Transfer the turkey to a flat surface and allow it to rest for 20 to 30 minutes before carving the meat.
Meanwhile, place the neck and the chicken stock in a pot and simmer gently until the stock reduces by about half. Season with salt and pepper.
Preheat the oven to 400ºF and grease a 9-by-13-inch baking dish with butter.
Cut the reserved turkey heart and gizzard into small pieces and set aside.
Melt 2 tablespoons butter in a large skillet over medium heat, then add the celery and onions and season with salt and pepper. Cook the vegetables, stirring occasionally, until translucent, 3 to 5 minutes. Remove the skillet from the heat and transfer the contents to a bowl. Set aside.
Arrange the bread slices in a single layer on a baking sheet and toast in the oven until light brown. While the bread is hot, lightly butter both sides of each piece, then cut into 1-inch squares and transfer them to a large bowl. Toss with the thyme, rosemary, sage and some salt and pepper. Mix and set aside. (Turn the oven down to 350ºF.)
Heat a tablespoon of the butter in a small skillet and quickly saute the heart and gizzard pieces, about 30 seconds. Sprinkle with salt and pepper, then remove from the heat and add it to the bowl of celery and onions. Stir to blend. Heat a large skillet, add another tablespoon butter and saute the pepperoni over high heat for 2 to 3 minutes. Drain on a paper towel.
Combine the celery and onion mixture, breakfast sausage, mozzarella and pepperoni in the bowl containing the toasted bread. Mix to blend, then add the chicken stock to moisten all of the ingredients. Transfer to the greased baking dish and bake for 40 to 45 minutes.
After removing the roasted turkey to rest, place the roasting pan over the burners of the stove over low heat and add the Marsala and mustard. Cook, scraping the bottom of the pan to get the drippings and tasty bits off, until there is almost no liquid.
Strain the neck out from the stock and pour about 1/2 cup into a small bowl. Whisk the flour into the bowl, taking care there are no lumps. Add the remaining chicken stock and the flour mixture to the roasting pan and whisk to blend. Taste for seasoning. Reduce until the mixture thickens to the desired consistency.
Carve the turkey and arrange on platters. Serve with the gravy on the side. (If the turkey is dry, ladle a little hot gravy directly on the meat.)