Scrub the mussels and remove the wiry little ’beards’ that are attached to the shells. Discard any open mussels that don’t close when given a sharp tap.
Place a large saucepan over a medium heat and add the butter and oil. When the butter is foaming, add the bacon or pancetta and cook for 4–5 minutes.
Stir in the leek, thyme and bay leaf and cook for 2–3 minutes, until the leek softens a little. Turn the heat up high, pour in the cider and bring to the boil. Stir in the mustard, then tip in the mussels and place a lid on the pan.
Steam the mussels for 2–3 minutes, giving the pan a good shake once or twice. When the mussels have nearly all opened (discard any that remain firmly shut), stir in the cream.
Season with salt and pepper and serve in deep bowls, scattered with parsley and accompanied by plenty of bread – or, even better, home-cooked frites – to soak up the sauce.
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