How to Make It
Preheat the oven to 350°. Set the turkey on a rack in a large roasting pan and scatter the turkey heart, gizzard and neck, the sliced celery, carrot, onion and garlic and the herb sprigs all around. Season the turkey with salt and pepper. Add 2 cups of the water to the pan and roast the turkey for 1 hour and 15 minutes. Add the remaining 2 cups of water to the roasting pan. Cover the turkey with foil and roast for about 1 hour and 15 minutes longer, or until an instant-read thermometer inserted into the thickest part of the inner thigh registers 170°.
Transfer the turkey to a cutting board and let rest for 30 minutes. Strain the pan juices into a large heatproof measuring cup, pressing on the solids. Skim the fat.
Pour the pan juices into a medium saucepan. Add 4 cups of the Turkey Stock and boil until reduced to 5 cups, about 5 minutes. In a bowl, whisk the flour with the remaining 1/2 cup of stock, then whisk the mixture into the stock. Boil, whisking, until the gravy is thickened. Carve the turkey and serve with the gravy.