2 loaves Italian bread (2 pounds), crusts removed and bread cut into 3/4-inch cubes (20 cups)
1/4 cup extra-virgin olive oil
1 1/2 pounds sweet Italian sausage, casings removed
2 large onions—1 finely chopped, 1 quartered
1 large celery rib, finely diced
3 large garlic cloves, very finely chopped
1/4 cup finely chopped sage leaves
4 tablespoons unsalted butter
6 cups turkey stock or low-sodium chicken broth
Salt and freshly ground pepper
One 18- to 20-pound turkey, rinsed
2 carrots, cut into 1-inch pieces
3 cups water
1/4 cup all-purpose flour
1. Preheat the oven to 375°. Spread the bread cubes in a large roasting pan and toast for 15 minutes, stirring occasionally, until dry and lightly browned.
2. In a large, deep skillet, heat the oil. Add the sausage and cook over moderately high heat, breaking up the meat, until browned and no trace of pink remains, 10 to 12 minutes. Add the chopped onion, celery and garlic and cook until softened, about 6 minutes. Stir in the sage and butter. Scrape the mixture into a large bowl. Add the bread cubes and toss. Stir in 2 cups of the stock and season with salt and pepper.
3. Increase the oven temperature to 450° and position a rack in the bottom of the oven. Set the turkey in the large roasting pan fitted with a shallow rack. Spoon 5 cups of the stuffing into the cavity and tie the legs together with kitchen string. Spoon the remaining stuffing into a shallow 2 1/2-quart baking dish. Scatter the quartered onion and the carrots around the turkey and add 1 cup of the turkey stock. Roast the turkey for 30 minutes. Lower the oven temperature to 375°. Roast the turkey for 3 hours longer, covering it loosely with foil as the skin browns and adding 1 cup of water every 45 minutes to prevent the pan from scorching. The turkey is done when an instant-read thermometer inserted into the thickest part of the inner thigh registers 175°. Transfer the turkey to a large cutting board and let it rest for 30 minutes.
4. Bake the remaining stuffing in the baking dish for 30 minutes, until the stuffing is heated through and crisp on top.
5. Meanwhile, pour the pan juices into a heatproof measuring cup. Spoon off the fat. In a small bowl, whisk 1/2 cup of the stock with the flour. Place the roasting pan over 2 burners on high heat. Add the remaining 1 1/2 cups of stock and cook, scraping up any browned bits stuck to the bottom and sides of the roasting pan. Pour in the reserved pan juices, then carefully strain the liquid into a medium saucepan. Boil over high heat until reduced to 4 cups, about 10 minutes. Whisk in the flour mixture and boil until thickened and no floury taste remains, about 5 minutes. Season the gravy with salt and pepper.
6. Spoon the stuffing from the cavity into a bowl. Carve the turkey and serve the stuffing and gravy alongside.