4 cups Turkey Giblet Stock, (recipe follows) or reduced-sodium chicken broth, divided
3 tablespoons all-purpose flour
1 1/4 cups apple cider
2 tablespoons cider vinegar
1/4 teaspoon salt
Freshly ground pepper, to taste
1. When you remove the turkey from the roasting pan, leave the roasted shallots behind. Skim off any visible fat from the pan juices.
2. Whisk 1/2 cup Turkey Giblet Stock (or chicken broth) and flour in a small bowl until smooth; set aside.
3. Set 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 3 1/2 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.
4. Whisk 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.