How to Make It
Preheat the oven to 400°. Set the turkey in a large roasting pan. Fold the wing tips under the bird and season the cavity with salt and pepper. Rub the bird generously with oil and set it breast side up in the pan; season with salt and pepper. Roast for 20 minutes.
In a bowl, combine 2 cups of the stock with the 2 tablespoons of oil. Baste the turkey with the stock mixture and scatter the onion, carrot and celery in the pan. Lower the oven temperature to 325° and roast for 31/2 hours longer, basting every 30 minutes and rotating the pan a few times, until an instant-read thermometer inserted in the inner thigh registers 165°.
Rub the turkey neck with oil and season with salt and pepper. In a saucepan, cook the turkey neck over moderately high heat until starting to brown, 3 minutes. Turn the neck, lower the heat to moderate and cook until browned, about 10 minutes. Add the remaining 6 cups of stock and bring to a boil. Cover and simmer over low heat, skimming, for 2 hours. Discard the neck; you should have about 41/2 cups of stock.
In a another saucepan, combine 3 cups of the tangerine juice, the ginger and brown sugar and bring to a boil over moderately high heat. In a small bowl, blend the cornstarch with the remaining 3 tablespoons of tangerine juice until smooth. Whisk the cornstarch mixture into the juice in the saucepan and bring to a boil, whisking until thickened, about 2 minutes. Remove from the heat, stir in the sage and season with salt; let cool.
When the turkey is done, remove it from the oven. Increase the oven temperature to 400°. Generously spoon half of the tangerine glaze over the turkey and roast for about 10 minutes, or until the skin starts to brown. Coat the turkey skin with the remaining tangerine glaze and bake for 10 minutes longer, or until richly browned. Transfer the turkey to a carving board and let rest for 15 to 25 minutes.
Meanwhile, set the roasting pan over 2 burners on moderately high heat. Add 1 cup of the turkey stock and boil, scraping up the browned bits. Strain the roasting pan juices into a medium saucepan, pressing down on the vegetables.
Skim the fat from the pan juices and add 1/4 cup of the fat to a bowl; discard the remaining fat. Blend the flour into the fat to make a smooth paste. Whisk the paste into the pan juices and bring to a boil, whisking constantly. Simmer until thickened, about 2 minutes. Whisk in the remaining 3 1/2 cups of turkey stock and simmer over moderately low heat, whisking often, until thickened, about 10 minutes. Carve the turkey and serve, passing the gravy.