This must-know technique is the basis for many sauces, soups, stews, and is a classic method of thickening.
The ingredients are butter and flour in equal parts. In a heavy sauce or sauté pan, butter is melted. Flour is added and stirred in rapidly, until the fat as been completely absorbed. Roux can be used immediately by adding liquid ingredients right away, or it can be stored in the refrigerator for later use.
There are varying degrees of roux, and certain recipes may specify dark (noisette), or light (blonde). The longer a roux is stirred over heat, the darker it becomes. This is because both the flour and the butter solids are cooking and carbonizing. The darker a roux is, the more delicious and toasty-nutty the flavor will be. ***However the longer it is cooked, the more the starch degrades, and therefore the less thickening power the roux will have.***
Some chefs use clarified butter, but I prefer the flavor that the butter solids impart. Some also insist on using high-protein flour, such as bread flour. I find all-purpose adequate for most roux, but I use bread flour when I need extra stability, such as in a soufflé recipe.
In case you are wondering, yes, the same technique can be done using any fat or oil. The flavors, of course, will vary. Some examples of alternative fats are bacon drippings as are often found in the early stages of stews; or olive oil, used in vegetarian renditions of classic recipes.