Making a rich, gravy-like Marsala sauce requires a little patience. When the sauce begins to boil, reduce the heat to medium and simmer for about 4 minutes, until the liquid reduces by half. If your sauce is not thick enough, simply combine 1 teaspoon of cornstarch (or arrowroot starch for a gluten-free version) with 1 teaspoon of the pan sauce in a small bowl. Whisk until smooth, we don't want a lumpy sauce, return to the pan and simmer for a few more minutes. Once it begins to boil it will thicken.