The Bird:
1 (14 to 16-pound) turkey, innards removed and reserved , neck reserved for gravy
1 large piece cheesecloth
1/2 stick unsalted butter, melted
Kosher salt and freshly ground black pepper
6 stalks celery, washed and sliced into 1-inch pieces
2 large onions, peeled, halved, and sliced
1 head garlic
2 sprigs fresh sage, stemmed and coarsely chopped
4 bay leaves
2 teaspoons dried rosemary

The Gravy:
7 cups low-sodium chicken stock
Salt and freshly ground black pepper
1 tablespoon Dijon mustard
1/2 cup dry marsala, or sherry
2 tablespoons all-purpose flour

The Stuffing:
1 stick unsalted butter, divided
1 pound loose pork sausage, broken into small pieces
6 cups 1-inch cubed sourdough bread, crusts and all