Pour pan juices from roasting pan, discarding any solids, into a fat separator. You should have 4 cups of pan juices, including 2 cups of fat. Pour pan juices into a medium saucepan and bring to a boil over medium heat, leaving fat in the fat separator. (If you do not have a fat separator, pour liquids into a tall quart-sized container and remove fat from liquid with a ladle.) In a quart-sized mason jar, combine chicken stock and flour. Shake well to combine. When pan drippings have come to a boil, slowly whisk in chicken stock mixture. Bring to a simmer. Continue to whisk until gravy thickens. Season with salt and freshly ground pepper. Keep warm until serving.