Cornstarch added directly to a hot liquid clumps and doesn't thicken properly. You can prepare many cornstarch-based recipes in the microwave, but some experimentation is necessary to ensure you can adapt your favorite stovetop recipes to microwave cooking successfully.
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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.
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Whisk in the remaining liquid for the recipe, typically broth or milk if you are making a sauce, soup base or gravy.
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Heat the mixture in the microwave for three minutes on the high power setting. Stir well after heating.
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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.
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Remove the bowl from the microwave promptly at the end of the cooking time. Stir in any remaining seasonings before serving.