Thicken the soup and give it a velvety finish with a simple addition
Beurre manié, a paste of equal parts soft butter and raw flour, thickens the soup without the toasting that roux provides.
It's important to distinguish beurre manié from roux, another French thickener made with butter and flour. Its name means "kneaded butter" and it uses unmelted butter mixed with raw flour, while roux uses melted butter and flour that is lightly toasted in a pan to eliminate the raw flour taste.
Butter acts as a carrier: it distributes the flour throughout the soup and helps prevent lumps. As you whisk and simmer, the flour absorbs the moisture, swells, and thickens the broth. As with cornstarch, adding too much flour at once will cause it to clump, and the lumps will retain a raw flour taste; therefore, it's best to add it gradually.
Unlike roux, which is added at the beginning of cooking, beurre manié is incorporated when the soup is almost ready. Just a minute is enough for the flour to dissolve, thicken, and lose its raw taste. If the soup hasn't thickened enough, add another small amount until the desired consistency is reached.
Beurre manié is an exceptional thickener for hearty soups, such as clam chowder, and a staple in some professional kitchens. To decide whether to use it, simply check the dish's ingredients: if they combine well with butter and flour on their own, this simple thickener is the right choice. Because it's easy to prepare, it can be kept on hand in the refrigerator, and it will keep well for up to a month in an airtight container.
It allows for variations: fresh herbs, salt and pepper, other flours, or different types of butter. Since the thickener doesn't cook for very long, it's best to choose ingredients that release their flavor quickly.
