Beurre Manie is equal parts flour and butter, and it's your new best friend for making soups and sauces.
Say you're making a soup or a stew, because it's cold and rainy out, and a soup is one of the best way to impart some much needed warmth into your bones. Your ingredients are all added, the flavor in the soup is where you want it, but the liquid isn't thick enough. You want it to coat the back of a spoon, and it's just...not there yet. Never fear. In my culinary techniques class, we learned that you always reduce liquid to develop flavor. You can always thicken it later. At this point you have a couple options for your soup, stew, or sauce, most liquids you deal with in cooking that you want to achieve a nice viscosity. You can use a roux, or a cornstarch slurry, certainly. Or you can use the easiest, best way to thicken up a soup, and that's a beurre manie.
The elements of a beurre manie are the same as a roux—all you need is equal parts fat and flour, in this case, butter. All you do is knead equal amounts of softened butter and flour together until they form a kind of thick, paste-like dough. Then you add small parts of the mixture, which should be room temperature or cold, to the hot liquid that you're trying to thicken, which should be simmering. Add a little bit of the beurre manie and whisk before you add more until you get the desired thickness—like any thickening method, adding it all at once can lead to a liquid that's overly viscous. The flour will thicken the liquid and the butter will add a nice finishing richness.
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Beurre manie is a remarkably effective way to thicken your dish without worrying about having a roux on hand. It's less limiting than cornstarch, which begins to release liquid after a while, and as long as you make sure to keep the temperature differential, the sauce or soup won't get grainy or lumpy. It's also a great way to quickly thicken up a gravy. You can also make it in advance, just like a roux—keep it in the refrigerator or until you're ready to use it.