Roux (Cooking Element)
A roux is a mixture of cooked flour and fat (usually butter), which is then used as a thickening agent for soups, stews, or sauces. A roux is classically (and best made) with an equal amount of fat and flour by weight, but you could also use volume measurements.
You start off by melting butter in a heavy pot over low heat. Once the butter is melted, sprinkle in the flour, constantly but slowly whisking it into the butter. Continue cooking the flour in the butter for at least 5 minutes to cook out the raw flour taste. The longer you cook the roux the darker and more flavorful it will become. Then you add the main liquid to the roux, or the roux to the main liquid, and bring it to a gentle boil to reach create the thickening of your sauce or soup. It’s best to stir as it comes to a boil to prevent the roux from sinking to the bottom and clumping.
From the French beurre roux (brown butter); Old French rous for reddish brown
Here are some dishes I’ve posted in which a roux is used: