1When you remove the turkey from the roasting pan, leave the roasted shallots behind. Skim off any visible fat from the pan juices.
2Whisk ½ cup Turkey Giblet Stock (or chicken broth) and flour in a small bowl until smooth; set aside.
3Set the roasting pan over two burners on medium-high heat. Add cider and vinegar; bring to a boil and cook, scraping up the browned bits from the pan, until the liquid is reduced by about half, 6 to 8 minutes. Add the remaining 3½ cups stock (or broth). Increase heat to high; return to a boil, whisking often. Boil until the liquid is reduced by about half, 8 to 12 minutes.
4Whisk the reserved flour mixture into the pan. Boil, whisking constantly, until the gravy is thickened, 1 to 3 minutes. Remove from the heat and pour the gravy through a fine sieve into a large measuring cup. (Discard the solids.) Season with salt and pepper.