From The Cook's Bible:
How to Thicken a Sauce

by Christopher Kimball

Step 1: Forget most of what you know about classic French cooking

If you have ever taken a course in classic French cooking, forget almost everything they told you. The French, with great intestinal fortitude, thickened sauces with butter, flour, egg yolks, and cream, to name a few of the culprits. They whisk together butter and flour (a roux) and then add in milk (a béchamel) or stock (a velouté), which thickens almost immediately. They use a paste, or little balls, of equal parts of flour and butter (beurre manié). These were added to sauces or stews as a thickener near the end of cooking. They also used egg yolks whisked into a sauce and reduced cream (cream that has been boiled down to thicken it).

Butter can also be used as a thickener. When it is whisked into a hot liquid, the milk solids and proteins in butter form an emulsion that suspends the particles of fat in the liquid, creating a thicker, shinier sauce. The problem with this last method, however, is that you need lots of butter to make this work (1/2 cup of butter is needed per 1 cup of liquid--1/2 cup of butter is an entire stick!). Nobody in their right mind is going to thicken with butter these days. Other thickeners include blood, foie gras (goose liver), yogurt, fresh cheese, and bread. Prior to the 16th century, coarse bread and ground almonds were the thickeners of choice.

So much for haute cuisine. The quickest and easiest thickener for a home cook is cornstarch. Whisk it with equal parts of water and then add it to a stock or a gravy and you have an instant sauce. This is how most people thicken gravy for Thanksgiving. But there are three good alternatives. Since most thickeners are starch, the two most obvious thickeners are potatoes and rice. Potatoes are particularly well suited to soups and stews (cook extra potatoes with the dish and then purée some of the liquid with the excess potatoes). I often keep extra cooked rice in the refrigerator as a thickener as well. I use it with stock and puréed vegetables for a quick and light sauce. I also use stale white bread, which is the traditional thickener for a rouille, a sauce made with roasted red peppers, lots of garlic, and hot peppers.

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