Special equipment: 1 large piece cheesecloth
For the turkey: Preheat the oven to 350 degrees F. Transfer the turkey to a roasting pan, fitted with a roasting rack if desired, and soak the cheesecloth in the butter. Season the bird with salt and pepper. Brush any remaining butter on top of the bird and cover the breasts with the cheesecloth to prevent the top skin from burning before it is cooked. Place the celery, onions, garlic, sage, bay, and rosemary in and around the turkey in the roasting pan - these will be used to make the stuffing when everything is roasted.
Place the roasting pan in the center of the oven. Cook for about 12 minutes per pound. After about 2 hours of cooking, remove the cheesecloth from the top of the breasts and return the turkey to the oven to cook for 1 more hour. If the tips of the wings start to get too dark, cover with foil to prevent them from burning.
For the gravy: Place the neck and the chicken stock in a pot and simmer gently on top of the stove as the turkey finishes cooking. The stock should reduce by about half. Season with salt and pepper.
How do you know when the turkey is done? The temperature of the thigh meat (where the meat is thickest and takes the longest time to cook) should register 165 degrees F when tested with a thermometer. When done, remove the bird from the oven, transfer it to a flat surface (or serving platter), and allow it to rest for 20 to 30 minutes before removing the vegetables for stuffing and carving the meat.
For the stuffing: Heat a large skillet and add half a stick of butter. Add the sausage and cook until brown and crispy. Reserve any grease and set the sausage in a large bowl with the roasted vegetables from the turkey tray. Add the remaining butter to the pan. Add the cubed sourdough bread and cook, stirring, until they toast golden brown, 6 to 8 minutes. Toss and season with salt and pepper before adding to the bowl with the vegetables. Stir everything together and taste for seasoning, adjusting if necessary.
For the gravy: Unless the bottom of the roasting pan is burned, you can make delicious gravy. Place the roasting pan over the burners of the stove, add the mustard and marsala to the pan, and warm it over low heat. Scrape the bottom to get the drippings and tasty bits off of the pan as the marsala reduces. 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. Reduce the marsala until there is almost no liquid. Add the remaining chicken stock and the flour mixture to the roasting pan. Whisk to blend. Taste for seasoning. Reduce until the mixture thickens. Transfer to a gravy boat.
Allow the turkey to rest for 15 to 20 minutes before carving. Serve sliced turkey with stuffing (dressing) on the side.