A roux is one of those unique cooking terms that pops up in many gourmet recipes, but isn’t really in the standard home cook’s cooking terminology. It is often used in Cajun cooking, but also in some soups, stews and sauces, even if they aren’t Cajun recipes.
A roux is a mixture of flour and cooking fat (varies including oil, fat from cooked meats, lard or even butter for a lighte roux, depending on recipe and what kind of base it is used for). It is used for both thickening as well as flavoring.
The mixture is approx 1:1 although many cooks use slightly more flour than fat/oil. Stir to remove lumps in the flour, if any.
The longer you cook a roux, the darker it will become. A light blond roux can be made over high heat in 4-5 minutes. A dark roux will take 20-30 minutes over high heat. Be sure to use a heavy saucepan. And don’t forget the heat of the saucepan will continue to darken the roux, so remove it from the heat slightly lighter colored than you need it.
You need to constantly stir your roux as you make it, as it can burn quite easily. This means standing and stirring constantly, not turning your back on it even for an instant.
To use a roux as a thickener, you need approx 1 tablespoon of your roux for each 1 to 1 1/2 cups of liquid/sauce.