It feels counter-intuitive to add water to a sauce in order to thicken it, but this trick works! As any Italian home cook will tell you, just a little starchy cooking water gives the sauce extra body and an almost creamy mouthfeel. Have you ever tried this?
All we do is dip out about a half a cup of the cooking water just before draining the pasta. It will look cloudy and yellowish from the starch. Stir this into your simmering sauce a few tablespoons at a time. The sauce will thin out a little and then thicken as the starchy water is absorbed.
This is less dramatic than adding cornstarch to a sauce or soup, but it works to bring the sauce together and make it silky. It works best with non-dairy sauces like tomato sauce, or with sauce-less pasta dishes like spaghetti carbonara or aglio e olio (pasta tossed with garlic and olive oil).
Give this a try next time you make a pasta sauce!