A roux is a cooked mixture of butter and flour. It helps to thicken a sauce but is incorporated at the beginning of the recipe rather than just before serving.
Pre-cooking flour allows the starch granules to swell and absorb moisture, and lets you thicken a sauce base without the flour clumping or forming lumps. A roux will deepen the colour of a sauce as well as giving it a nutty, toasted flavour.
To make a basic roux, use equal weights of butter and flour. If you don’t own a kitchen scale, use tablespoons. One tablesspoon flour and one tablespoon butter.
Melt the butter over low to medium heat; slowly add the flour to the butter, whisking constantly.
Within 2 to 3 minutes the roux will have a consistency of a cake frosting. A white roux is done when the flour loses its “raw” smell and begins to develop a toasty aroma.
Darker roux are cooked, stirring constantly, until the desired colour.
Immediately remove the pan from the heat and transfer the roux to another container to cool.