Separate the parsley leaves from the stalks, and chop the leaves (hold on to the stalks!).
Put the milk, onion, bay leaf and parsley stalks into a medium-sized saucepan, and heat until it’s just about boiling.
Turn off the heat and leave the mixture to stand for 25 minutes, then strain it through a sieve (or take the bits out with a slotted spoon). Make sure the strained milk is nearby, as you add it in later.
Melt the butter in a non-stick pan, and stir in the flour – this will turn into a paste (known as a roux).
Cook this gently over a low heat for 3-4 minutes – and don’t stop stirring!
Now add some of the strained milk, a bit (a short pour) at a time. Keep stirring as you add it in, and it’ll slowly turn into a thick sauce.
Simmer the sauce gently for 10-15 minutes.
Add the chopped parsley leaves and the lemon juice. Try a little bit (mind the heat), then add salt and pepper to season it.
Plate up parsley sauce with grilled fillets of white fish, new potatoes and steamed green vegetables for a simple yet elegant dinner.