Gather the ingredients.
In a medium-sized saucepan, melt the butter over a medium heat.
Stir in the flour and mustard if using.
Stir thoroughly and a thick paste will form but don't worry.
Cook gently for a further 2 to 3 minutes, watch the heat to ensure sure the paste does not burn.
Gradually stir in the milk. I prefer to use a whisk, as I find it easier, but a wooden spoon is fine.
Bring to the boil, lower the heat and simmer for 5 minutes, stirring frequently to make sure there are no lumps. The sauce should be quite thick but still a pouring consistency, if too thick add a little more milk.
Add the parsley and stir well.
Add a good pinch of sea salt and a few grinds of black pepper, taste and add more to taste.
Keep the sauce warm until needed. If you need to keep the sauce longer than about 15 minutes I recommend laying a piece of buttered greaseproof paper on the surface to prevent a skin forming. If a slight skin does form, whisk thoroughly and it should disappear.
Serve with fish or ham and enjoy!