Heat the fat in a 3-quart saucier over medium-high heat. When the bubbles begin to subside (a sure sign that most of the water has cooked out), add the flour all at once and whisk 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.
Remove from the heat and let cool a minute or two before using. Tightly wrapped roux can be refrigerated for up to a week or frozen until the end of time.
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.