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Beurre manié
From Cookipedia
Beurre manière (French "kneaded butter") is a dough, consisting of equal parts of soft butter and flour, used to thicken soups and sauces. By kneading the flour and butter together, the flour particles are coated in butter. When the beurre manière is whisked into a hot or warm liquid, the butter melts, releasing the flour particles without creating lumps.
Beurre manière should not be confused with roux, which is also a thickener made of equal parts of butter and flour, but which is cooked before use.
