Combine 1 part cornstarch with 2 parts cold water in a microwave-safe bowl. Use ½ tablespoon cornstarch for every ½ cup of liquid you are thickening. Whisk the cornstarch and water together until the starch is dissolved in the water and the water looks milky.
Whisk in the remaining liquid for the recipe, typically broth or milk if you are making a sauce, soup base or gravy.
Heat the mixture in the microwave for three minutes on the high power setting. Stir well after heating.
Cook at high power for an additional two minutes, or until the mixture begins to boil. Allow the mixture to boil for one full minute.
Remove the bowl from the microwave promptly at the end of the cooking time. Stir in any remaining seasonings before serving.