Roux is a mixture of butter and flour, cooked together. It is used as a thickening agent for sauces, soups, and gravies.
To make a roux, melt butter in a saucepan over medium heat. Add an equal amount of flour and whisk until smooth.
Cook the roux for a few minutes, stirring constantly, until it reaches the desired color. A white roux is cooked for 1-2 minutes and is used for béchamel sauce. A blond roux is cooked for 3-5 minutes and is used for velouté sauce. A brown roux is cooked for 10-15 minutes and is used for espagnole sauce. A dark brown roux is cooked for 20-30 minutes and is used for demi-glace.