Roux method: Melt the butter in a saucepan over medium-low heat. Stir in the flour to make a roux. Whisk in a little of the milk at a time, stirring continuously until you have a smooth, slightly thick sauce. Remove from the heat; season with salt and pepper. If you want to make it a cheese sauce, then add the cheese while it’s still hot.
All in one method - inspired by Delia: Add the milk, butter and flour to a saucepan and warm through on a low/medium heat to melt the butter. Once everything has melted then turn the heat to low. Keep whisking the whole time until the sauce starts to thicken and becomes smooth and glossy. Once the sauce has thickened continue to cook (and whisk) for a further 5 minutes.
