A quick and effective last-minute technique for thickening a stew is what’s known as beurre manié (pronounced burr mahn-YAY), a French term that translates as kneaded butter. Despite its fancy-sounding name, beurre manié is nothing more than a soft paste made of equal parts butter and flour that you whisk into a simmering liquid just before serving.
The flour’s starch is what thickens the liquid, but if the flour weren’t first thoroughly combined with the butter, it would quickly lump up. The butter actually coats the particles of flour so that they can disperse evenly in the simmering liquid. The butter also adds a touch of richness, unlike the thin flour-and-water paste called a slurry or a whitewash.
To make beurre manié, knead butter and flour together with your fingertips or a wooden spoon until well combined. Make sure the butter is somewhat softened (but not melted or oily) before kneading.
There is no exact formula for how much beurre manié you need to thicken any given amount of liquid. It will depend on how thick the stew is to begin with and on your personal taste. For a good-size pot of stew (about 3 quarts or so), I generally measure out 2 or 3 tablespoons of butter into a small bowl or onto a saucer and then knead in an equal volume of flour until thoroughly combined.
Whisk in the beurre manié a bit at a time. Right before serving, bring the liquid to a vigorous simmer and then whisk in an acorn-size hunk of beurre manié. Let the liquid return to a simmer, whisking continually, and gauge its viscosity—beurre manié will take effect instantly as soon as the liquid returns to a boil. Continue whisking in small bits of beurre manié until you get the thickness you’re after. But be conservative since, unlike roux-thickened liquids, a beurre manié can leave a sightly floury taste.
Once thickened, the stew should be served immediately or set aside off the heat for later. Extended simmering can undo the effects of a beurre manié and bring out a floury note.