In a pan over moderate heat, melt and blend butter or other fat with flour. How long you cook roux before adding liquids depends on what you are making. For example, for dark gravy you would cook until rich brown in colour. For white sauce, cook it just a few minutes, until still light in colour. 1 Tbsp. flour mixed with 1 Tbsp. of butter or other fat should yield enough roux to thicken 3/4 to 1 cup of warm liquid. To avoid lumps forming, slowing whisk liquid into the roux and simmer until mixture thickens.

Instead of cooking it, you simply mix fat and flour into a paste in a bowl. The paste is then gradually whisked into simmering liquids that require thickening. 1 Tbsp. flour mixed with 1 Tbsp. of fat should yield enough beurre manié to thicken 3/4 to 1 cup liquid.

For example, to thicken three cups of simmering stock to make gravy, combine 1/4 cup (for thinner gravy) or 1/3 cup (for thicker gravy) all-purpose flour in a bowl with 1/2 cup water or stock. Whisk this mixture into the pan and simmer until gravy thickens and raw flour dissipates, about five minutes.