Cornstarch gives silkiness and body to a thin gravy or sauce. Use the wrong technique to add it, and you end up with a lumpy, sticky mess. To avoid this culinary disaster, create a slurry by dissolving the starch in a small amount of cold liquid before adding it to the entire dish. The cold liquid could be water, milk or broth — whatever complements the dish. Stir the slurry thoroughly, until no lumps of cornstarch remain. Pour the entire slurry into the dish, while stirring, and cook at a simmer for just a minute or two to thicken the sauce and cook out any starchy taste.
A tablespoon of cornstarch generally thickens one cup of liquid. Be careful, though, because too much cornstarch makes your sauce gelatinous; you’re better off adding too little than adding too much. Avoid freezing sauces that you’ve made with a cornstarch slurry — they turn gritty when reheated. Add the slurry only in the last minute of cooking; cook it too long and your sauce turns gluey.
References and ResourcesCook's Thesaurus: Starch Thickeners
The Kitchn: How to Make and Use a Slurry to Thicken Soup
Serious Eats: Seriously Asian: The Function of Cornstarch