A rich béchamel sauce, a classic clam chowder, and a spicy gumbo may not seem to have much in common. Yet all three are thickened with a cooked mixture of flour and fat known as roux. Roux (pronounced “roo”) is one of the cornerstones of sauce-making and it’s made in three versions: white, blond, and brown (see photos at right).

Most recipes call for equal weights of fat and flour, but making roux is not an exact science. Experienced cooks often add the flour until it looks right. A colleague describes perfect roux as “wet sand at low tide”: moist but not runny.

As a roux cooks, it gets darker and its flavor becomes more complex. It’s important to understand, however, that as a roux colors, it loses its ability to thicken because the starch in the flour is broken down by the heat. You’ll need more brown roux than blond roux to thicken the same amount of liquid. A truly dark roux won’t thicken at all.

How to Make a Roux

Begin by heating the fat. Butter is used most often, but different tastes and traditions call for different fats, including vegetable oil, clarified butter, lard, or duck fat. Once the butter is melted, add the flour and stir until smooth. Coating the flour with fat prevents it from forming lumps when mixed with a liquid.

Cook roux over medium-low heat and stir constantly to prevent scorching. High heat will burn a roux, making it grainy and off tasting.

Adding Liquid to a Roux

Once a roux is cooked to the proper color, gradually add the liquid your recipe calls for, whisking constantly. I prefer to warm the liquid slightly first. If it’s too cold, you’ll spend extra time whisking while the sauce comes to a simmer. But if the liquid is too hot, the sauce may thicken so quickly that it becomes lumpy. Bring the mixture to a gentle simmer, skimming any foam or fat that rises to the surface.

Long cooking improves both the flavor and texture of the sauce.A roux-thickened sauce should simmer for at least 30 minutes. As the liquid reduces and thickens, the pasty taste of raw flour disappears, and the sauce becomes rich tasting and velvety smooth.