Heat fat or over medium high heat. Add flour all at once whisking vigorously. When mixture thins and starts to bubble, reduce heat to low and cut back on the whisking. Cook until you smell a toasty aroma then cook 2 minutes more, stirring occasionally.
Roux can be used immediately to thicken a liquid that is at or below room temperature. To thicken a hot liquid, allow roux to cool to room temperature, or refrigerate.
Tightly wrapped, roux can be refrigerated for up to a month. Simply break off pieces and use as needed.
Recipe courtesy of Alton Brown