How to make a roux

Melt 225g butter in a pan over medium heat.

Gradually add 225g plain flour, whisking between each addition.

A thick paste will form.

Continue to whisk constantly for 5 minutes until the paste whitens and becomes thin and smooth.
A white roux is cooked just until the flour loses its raw smell and is used most often to thicken milk-based sauces, such as white sauce and cheese sauce.

After 20 minutes of continuous stirring, the mixture will darken. This is called the 'blonde' stage.
Also sometimes called 'golden' roux, this roux lends a nuttier flavour and provides excellent thickening power. It's multi-purpose, used in soups, stews and stock-based sauces.

After 35 minutes of continuous stirring, the mixture will darken further and enter the 'brown' stage.
Brown roux actually has less thickening power than a blonde roux. It has a more pronounced nutty aroma and is used in Cajun dishes like gumbo.

After 45 minutes of stirring, the roux will darken further.
With the least amount of thickening power, a dark brown roux has a mellower flavour than brown roux. It's main use is as a flavouring agent, with thickening being secondary.

Avoid lumps To ensure lump-free thickening when making sauces, such bechamel, the liquid ingredient should be cold or room temperature, and slowly whisked into the hot roux.
Make ahead You can make a large batch of roux and store it in the fridge to use as and when needed. Simply pour the cooked roux into a heatproof container and allow to cool. As it sits, the flour will begin to settle to the bottom, and the fat will rise to the surface. Stir the fat back into the flour before using. After the roux has cooled, transfer it to an airtight container and refrigerate. Roux will keep in a sealed container in the fridge or freezer for 4 to 6 months, until ready for use.
Small batches When measuring by weight, use a 1:1 ratio and simply measure the same weight of fat and the same weight of butter. If you're only thickening 500ml of liquid, for example, you may just want 15g of butter and 15g of flour (equivalent to 1 tablespoon of butter and 2 tablespoons of flour).
Non-dairy roux Most people think only butter can be used in a roux. However the very same technique described above can be used with a variety of other fats, from lard to margarine to olive oil. This is especially handy information if you're making vegan or allergy-friendly soups or sauces.