Meanwhile, strain the pan juices into a large measuring cup; discard the solids. Spoon off the fat and discard it. (You should have about 2 cups of defatted pan juices.) In a large saucepan, boil the turkey stock until it is reduced to 3 cups, about 15 minutes. Set the roasting pan over 2 burners on high heat; add the reduced stock and scrape up any browned bits stuck to the bottom and sides of the pan. Strain the stock into the saucepan. Add the pan juices and bring to a simmer. Whisk in the reserved butter-and-flour mixture and simmer over moderate heat, whisking constantly. Cook until thickened, about 4 minutes, and season the gravy with salt. Add the remaining 2 tablespoons of chives. Carve the turkey and serve, passing the gravy at the table.