Pour the milk in a saucepan, then simply add the plain flour, butter and seasoning and bring everything gradually up to simmering point over a medium heat.
Whisk continuously with a balloon whisk, until the sauce has thickened and becomes smooth and glossy.
Turn the heat down to its lowest setting and let the sauce cook very gently for 5 minutes stirring from time to time. This takes the yucky taste out of the flour.
Add the chopped parsley, cream and lemon juice, taste and check for seasoning, then serve in a warmed jug.
Place in the middle of the table so everyone can help themselves.