Discard any onion skins from the turkey roasting pan and pour the drippings and solids into a 1-qt. measuring cup. Let the liquid settle (throw in a few ice cubes to speed up the separation process, if desired). Skim off and discard the fat from the surface. You should have 2 to 2 1/2 cups of defatted drippings (add stock or broth, if necessary, to equal 2 1/2 cups). Return the pan drippings and solids to the roasting pan.
Place the flour in a medium bowl. Using a fork, gradually stir in 1 cup turkey stock. (If the stock is hot, add it very slowly to prevent lumps from forming.) Gradually mix in 1/2 cup more stock.
et the roasting pan on 2 burners over medium-high heat. Scrape up any browned bits at the bottom of the pan and bring the drippings to a simmer. Whisk in the flour paste and bring to a simmer, whisking constantly. Gradually whisk in the remaining 2 1/2 cups turkey stock and boil, whisking, until thickened, about 12 minutes. Season to taste with salt and pepper.