First place the milk in a small saucepan and add the parsley stalks, bay leaf, mace (if using), peppercorns and onion. Then place it over a low heat and let it come very slowly up to simmering point, which will take approximately 5 minutes. Then remove the saucepan from the heat and strain the milk into a jug, discarding the flavourings.

Take a washed pan and place it over a gentle heat. Begin by melting the butter gently - don't over-heat it or let it brown, as this will affect the colour and flavour of the sauce. As soon as the butter melts, add the flour and, over a medium heat and using a wooden spoon, stir quite vigorously to make a smooth, glossy paste. Now add the infused milk a little at a time and whisk well with balloon whisk till milk is incorporated and sauce look smooth, glossy and creamy.

Now turn the heat down to its lowest setting and let the sauce cook for 5 minutes, whisking from time to time. While that's happening, taste and season with salt and freshly black pepper. If you wish to keep the sauce warm, all you do is pour it into a warmed jug and cover the surface with clingfilm.