Roux is a mixture of equal parts flour and fat, heated so that the flour toasts and as a result can be used as a thickener in soups, stews, and stocks without clumping. There are several types of roux depending on how long it is cooked ranging from blonde to dark.
When adding roux to a liquid the liquid must be brought to a boil for the thickening power of the roux to be fully activated. It's wise to make roux apart from the liquid preparation and add in roux gradually and as needed. The liquid with the roux should then be simmered for at least 15 minutes to get rid of any flour taste in the final result.
To make blonde roux, weigh out equal parts flour and fat, traditionally neutral oil or butter. wisk the two ingredients together in a pot while over moderate heat. Whisk continuously until you arrive at a blood color that smells slightly toasted. Remove it from the heat to stop further cooking. Blonde roux is the most common type of roux and in general is what you use to thicken soups.
Brown roux is made the same as blonde roux except that it cooks for a longer period of time to further toast the flour and further develop a nutty flavor from the flour. It is famously used in sauce espagnole, one of the five French mother sauces. Espagnole is brown stock thickened with a brown roux.
Roux can be further darkened to the color of a rusty penny as in the case of the deeply dark roux that is added to gumbo. Special attention needs to be taken to not burn dark roux. if you scorch a roux, it should be discarded and you should start a new one or risk your soup tasting burnt.
An alternative to roux is beurre manié, "kneaded butter" in French. unlike roux, beurre mainé is not cooked It is kneaded together and the mixture is used to finish soups and sauces with a velvety sheen.