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Melt the butter in a pan over a medium-low heat, then stir in the flour. Cook, stirring, for 1-2 mins until a paste forms.
Gradually pour in the milk, stirring continuously until all the milk has been incorporated and the sauce coats the back of a spoon. If the sauce is lumpy, whisk for a few minutes more, or strain through a sieve and return to the pan. Bring to a simmer and bubble for 1 min. Remove from the heat and stir through the parsley. Add the lemon juice and mustard to taste, if using. Season well with salt and freshly ground black pepper. Once cool, will keep chilled for up to three days or frozen for a month. Defrost overnight in the fridge and reheat over a low heat.