In a heavy saucepan, melt the butter, stir in the flour, and cook for a few minutes.
Remove from the heat, and stir in about a quarter of the milk until smooth. Put on the heat again, stirring continuously, and, as the mixture begins to thicken, remove again from the heat.
Gradually add another quarter of the milk, stirring until smooth, thickening it again over the heat, and so on, until you have a smooth thick sauce. Season lightly with salt and white pepper, if making a savoury sauce. If you do not mind the darker flecks, a grating of nutmeg is good too.
Cook it gently for 10 minutes. This is important. Even though the sauce looks finished before then, it is important to fully cook the flour.
To make cheese sauce, to the above quantity of sauce, stir in about
For a richer or sweet sauce, substitute single cream for the milk. Sweet sauces can be flavoured with grated lemon or orange zest, and sweetened with sugar or honey.