Now make your sauce. Melt the butter in a pan, add the flour, and stir on a gentle heat—do not let it color, it’s ready for the milk when it smells biscuity. Add the milk, whisking ferociously, making sure the heat’s not too fierce. When you have a firm white creamy mixture, add a ladle of ham stock, and whisk again. Do this until you have reached your desired consistency. Test for seasoning. Just before serving add the chopped parsley and stir.