After you roast the turkey, place the roasting pan on the stove top 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 add several ice cubes to the liquid and placing it in the freezer for 10 minutes. Remove hardened fat with spoon and place stock into a saucepan. You should still have about 4 cups of liquid.
Place the saucepan on the stove top over medium heat and bring to a simmer. Add sage and thyme to the simmering stock. Continue to simmer until the stock is reduced by 1/4, 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.