Tomato-Based Sauce

• To thicken a tomato-based sauce, allow it to slowly cook in an uncovered pot over low heat. Excess liquid evaporates as the sauce simmers, leaving a thicker sauce over time. You can speed things up by adding a tablespoon of tomato paste at a time into the simmering sauce, stirring until it's incorporated. Be sure to taste the sauce before each addition, because too much tomato paste can make the sauce acidic and overwhelm the flavor of the herbs in the sauce.

White and Other Sauces

• White spaghetti sauce, or salsa di besciamella, starts with a roux of butter and flour, stretched with hot milk. A higher flour and butter-to-milk ratio, such as 3 tablespoons each of butter and flour to 1 cup of milk, should make the sauce thick. To thicken a runny sauce, add more flour or even instant tapioca, or stir in cream cheese, heavy cream, grated cheese or cooked ground beef to absorb some of the excess liquid. The best way to thicken a sauce is to cook it longer, but be especially careful with a white sauce, as it can catch on the bottom and burn more easily.