Set the roasting pan with the turkey drippings and vegetables over two burners set on medium high. Add the Cognac, vermouth, and 1/2 cup of the turkey broth; cook, stirring with a wooden spoon or wooden spatula to loosen the browned bits in the pan, until the liquid comes to a simmer. Strain the contents of the roasting pan through a large sieve and into a large saucepan. Add the remaining 2 cups turkey broth and the thyme to the saucepan and bring to a boil over medium-high heat; reduce the heat and let simmer to blend the flavors, about 5 minutes.
Meanwhile, put the cream in a small bowl and whisk the flour into the cream to make a smooth paste. Gradually whisk the cream mixture into the turkey broth mixture. Bring to a boil over medium-high heat, reduce the heat to low, and gently simmer to thicken the gravy and cook off the raw flour flavor, about 10 minutes. Keep hot until ready to serve.
This recipe and photo are courtesy of Fine Cooking