A roux is made by combining equal amounts melted butter and flour with a liquid. Recipes will specify the exact quantities but the golden rule is to use equal amounts of flour and butter. Melt butter in a heavy-based saucepan over a moderate heat.
Add flour all at once; blend in, stirring constantly for 3 to 10 minutes (see below). Remove from heat and gradually add some of the liquid. Return to heat; whisk constantly until all the liquid is absorbed and sauce is smooth and lump-free. Continue until all liquid is used, boiling and thickening sauce after each addition.
The colour of the roux is determined by how long the flour is cooked; roux blanc is white and takes about 3 minutes; roux blond is a pale, sandy colour and takes about 5 minutes; roux brun is cooked about 10 minutes until it becomes brown.