Melt the butter in a saucepan, then add the plain flour. Stir continuously until a paste forms – this is called a roux. Continue cooking for 2 mins. The roux needs to be kept on the heat for at least 2 minutes to cook the raw flour.

Depending on what sauce you are using the roux to make you want to start adding hot liquid to the roux a bit at a time, whisking well between each addition so your finished sauce is smooth then leave to simmer gently, stirring occasionally, for 10 mins until your sauce is thick and velvety.