Your chicken and vegetables are bubbling away and your dough is ready for a final assembly as a potpie. But before you can pour all that goodness into the crust, you have to transform the broth into a thicker sauce that won't leak out during baking. The simplest way to do this is to stir in a blend of 1 part cornstarch and 1 part water, which will thicken the liquid to gravy consistency without altering the flavor.
When working with a cornstarch and water blend to bulk up a soup, stew, sauce or gravy, it's best to start with small amounts depending on how much broth or stock you need to thicken. While you can always thin a sauce that has gotten too thick, any additional liquid could throw all the other ingredients off and ruin the dish. The ratio of cornstarch to the amount of liquid you want to thicken is about 1 tablespoon to 1 cup for a thin to medium sauce and 2 tablespoons to 1 cup for a thicker gravy.
Cornstarch is most readily absorbed by a hot liquid if it is first mixed with a small amount of cold liquid, usually water. A ratio of 1 tablespoon cornstarch to 1 tablespoon cold water or stock results in a thick white paste called a slurry that is easily incorporated into the stew and does not create any lumps. Stir the slurry in slowly, and cook about one minute. If the stock shows little to no sign of thickening by that time, stir in another batch of slurry made with the same ratio of cornstarch to water.
If making gravy from pan drippings, pour off all but about 2 tablespoons of the drippings, and place the pan over low to medium heat. Stir in a slurry made from 2 tablespoons cornstarch and 2 tablespoons water, and blend well into the drippings. When a thick brown paste has formed, it's time to add whatever liquid you are using, which can be more water, stock or broth made using bouillon cubes dissolved in water. Whisk the liquid into the paste slowly until it thickens and remove from heat. You can also add flavorings to the slurry to thicken the sauce or gravy for Asian-inspired dishes, such as teriyaki sauce or soy sauce.
If you aren't sure how much broth is in your soup or stew, start with the smaller amount and add more cornstarch until it is the thickness you want. If working with a small amount of liquid, such as a gravy made from pan drippings or stock, 2 tablespoons cornstarch blended with 2 tablespoons cold water will thicken about 2 to 3 cups of gravy. The thickening effect usually happens within two minutes if enough cornstarch has been used, and the stock or soup should be simmering gently to allow for complete distribution of the slurry. If your slurry has separated from sitting too long, simply stir it again to combine the cornstarch and water.