After you roast the turkey, place the roasting pan on the stovetop over medium heat. Add 2 cups of turkey stock to the pan and stir about 5 minutes or until the drippings and browned bits from bottom dissolve.
Place a strainer over a fat separator cup. Pour pan drippings through strainer. Add enough stock to the drippings to make 4 cups total.
If you don't have a fat separator cup, you can remove fat from drippings by adding several ice cubes to the liquid and placing it in the freezer for 10 minutes. Remove hardened fat with a spoon and place stock into a saucepan. You should have about 4 cups of liquid.
Place the saucepan on the stovetop over medium heat and bring to a simmer. Add the sage and thyme. Continue to simmer until the stock is reduced by one-fourth, or until about 3 cups of stock remain.
Pour milk into a small bowl. Add cornstarch and stir to mix evenly. Slowly pour milk mixture into the simmering stock, stirring slowly. Bring sauce to a boil and continue to stir until stock thickens and has a nice shine, about 3 to 5 minutes.
Pour gravy into warmed gravy boat and serve.