How Do You Make A Parsley Sauce?

The following is a recipe for parsley sauce.

Put 425ml of milk a few parsley stalks, 1 bay leaf, 1 slice lemon (about 5mm thick), 1 blade of mace and 10 whole black peppercorns into a small pan and slowly bring everything up to simmering point.  

Pour the mixture into a bowl and leave it to get completely cold.

Strain the milk back into the pan and discard the flavourings.  Add 20g of plain flour and 40g of butter, and bring it all gradually to simmering point, while whisking continuously with a balloon whisk.  Do this until the sauce has thickened and is smooth and glossy.

Turn the heat to its lowest possible setting and cook the sauce gently for about 5 minutes  while stirring it from time to time.  

To serve, add 4 heaped table spoons of finely chopped parsley, 1 tablespoon of single cream and 1 teaspoon of lemon juice, along with salt and black pepper that has been freshly milled.