Heat the butter in a deep, wide pan over a medium heat. Add the onion and garlic,then cover and sweat for 5 minutes or so, stirring occasionally, until softened but not coloured.
Increase the heat and add the thyme. When it releases its scent, add the cider vinegar,if using, and cider, then the mussels and some salt and pepper.
Give them a quick stir and a shake, then cover with a tight-fitting lid.
Cook for 3–4 minutes, shaking the pan a couple of times, until the mussels are open (discard any that remain closed). Add the cream, if you like, to the juices in the pan.
Spoon the mussels into warmed bowls, pour the juices over and around them, and serve with good bread and a glass of cider.
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