A roux is just equal parts fat and flour cooked together, but it is the backbone of some of the most useful sauces in the kitchen.
Melt butter over medium heat, whisk in an equal weight of flour, and cook for a minute or two to lose the raw-flour taste. Cook it longer for a deeper, nuttier colour and flavour.
Then whisk in your liquid gradually — warm milk for a béchamel, stock for a velouté — adding a little at a time to keep it smooth. Simmer until it thickens, season, and you have the base for cheese sauce, gravies, pies and gratins.
