Beurre manie is one of the best ways to thicken a sauce or a soup, period. This fancy-sounding mixture—it means kneaded butter in French—is incredibly simple to make and equally easy to use. Just rub enough flour into softened butter to make a thick paste; then whisk in little bits of the paste to finish a pan sauce for, say, shrimp scampi or a roast turkey, or to enrich a seafood chowder. As the butter melts, it separates and evenly disperses the flour particles, which swell and thicken the liquid. The result: a lustrous, velvety texture with nary a clump. Once a technique that was employed by professional and home cooks, unfortunately, this smart kitchen trick is rarely seen anymore. We think it's time to revive it.