I use Delia's parsley sauce recipe, though I don't use the mace very often, as I usually don't have any! and I don't bother with the extra cream.
Ingredients
15fl oz/425ml milk
a few parsley stalks
1 bay leaf
1 slice onion, ¼in/5mm thick
1 blade of mace
10 whole black peppercorns
¾oz/20g plain flour
1½oz/40g butter
4 heaped tbsp finely chopped fresh parsley
1 tbsp single cream
1 tsp lemon juice
salt and freshly milled black pepper
Put the milk parsley stalks, bay leaf, onion, mace & peppercorns in a small pan, bring slowly to simmer, then pour the mixture into a bowl and leave aside to get completely cold. (I rarely do this as I'm usually making it at the last minute, doesn't seem to make a lot of difference to me!)
When you're ready to make the sauce, strain the milk back into the pan, discard the flavourings, add the flour and butter and bring to simmering point, whisking continuously with a balloon whisk until the sauce has thickened and is smooth. Turn the heat right down & cook gently for 5 minutes, stirring from time to time.
To serve the sauce, add the chopped parsley, cream and lemon juice, taste and season, then serve in a warm jug.