Cornstarch gives silkiness and body to a thin gravy or sauce.
For each cup of liquid you want to thicken, start with 1 tablespoon of cornstarch in a small bowl. Add an equal amount of cold liquid (water, milk or broth) and stir until smooth paste forms.
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. Too much cornstarch will make 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 as they turn gritty when reheated. Add the slurry only in the last minute of cooking. The sauce will turn gluey if you cook it too long.