Roux is a mixture of flour and fat used as a thickener, and is an essential building block in many sauces and soups. It acts as the thickener in three of the mother sauces, espagnole, béchamel and vélouté. Without a roux, neither macaroni and cheese nor gumbo would retain their trademark thick, rich texture.
When it comes to making and using a roux, there are a few basic rules. A roux’s basic ingredients are flour and a fat. The most common fat used is clarified butter, but oil, chicken fat (schmaltz), fat from a roast (think Thanksgiving gravy) or plain butter can also be used. Keep a 1:1 ratio of fat to flour otherwise the roux could be too thick and prone to creating lumps or, conversely, too runny and will not give the thickening power necessary for the recipe.
The amount of time a roux is cooked before the liquid in the recipe is added determines the type of roux produced and its thickening ability. Cooking a roux toasts the starch in the flour, which adds a nutty flavor, but also diminishes the roux’s ability to thicken. Cooking a roux slowly helps control the type of roux you make- white, pale or brown. Cajun cooking uses a dark brown roux made by either cooking for a long period of time or adding the oil and flour to a very hot pan.
When adding the liquid to a roux, have your whisk handy. The rule is to add hot liquid to cold roux or vice versa. Whichever way you choose to go, you have to whisk vigorously to ensure a smooth, lump-free sauce. Also, you have to cook the flour, starchy flavor out of a roux. It can either be done by cooking the roux by itself for a few minutes or by cooking the final sauce or soup itself.