When cool enough to handle tip any juices from the cavity of the bird into the roasting pan and set bird aside on a cutting board to cool, tented with foil. Set the roasting pan and accumulated juices over two medium flames, add 1 cup of water and scrape up any browned bits from the pan.
Carefully strain out the broth into a large glass measuring cup or medium glass bowl and place in the refrigerator to separate the fat from the broth. You should have about at least 2 cups of broth.
Once cooled down, the fat should rise to the top and you should be able to separate most of it out. Reserve 3 tablespoons of fat and discard the rest. Measure out the remaining broth and add turkey stock until you get 4 cups total of liquid.
In a medium sauce pan add 3 tablespoons fat and whisk in cornstarch. Slowly whisk in broth/stock until it is nice and smooth and bring to a boil and cook for 5 minutes. Season to taste with salt and pepper.