Heat the stock in a large pot over medium heat. Add the turkey neck and bring to a simmer. Turn down the heat to low and let simmer for 20 minutes.
Unless the bottom of the roasting pan is burned, you can make delicious gravy. Place the roasting pan that you used to roast your turkey in over the burners of the stove, add the marsala and mustard to the pan, and warm it over low heat. Scrape the bottom of the pan to get the drippings and tasty bits off as the marsala reduces. Reduce the marsala until there is almost no liquid left in the roasting pan.
Remove the neck from the stock. Use a small strainer to sift the flour into a medium bowl. Slowly add 1 cup of stock into the flour, whisking constantly to blend smooth and lump free.
Whisk in the flour mixture and remaining chicken stock to the roasting pan. Reduce until the mixture thickens. Generally, a gravy is a thick sauce that is made fairly quickly. Cooking it for a while longer can help the starch in the flour break down and yield a slightly smoother texture. If any lumps, strain before serving. Transfer to a gravy boat.