Start by frying the pancetta in a large casserole dish over a high heat. Finely chop the onion, and when the pancetta is starting to turn golden, add the onion to the dish. Turn the heat down and sweat the onion for 2-3 minutes.
Pour in the cider and 200ml water, turn the heat to low and allow it to simmer while you prepare the mussels.
To prepare your mussels, take each mussel from the bag one at a time. If the mussel is closed, pull off the ‘beard’ (the hairy strands which poke out from the shell) and gently scrape off any barnacles. Place in a large colander. If any are open, tap them gently. If they don’t close, discard them. If they close, again remove the beard and add to the colander. If there are any with smashed or cracked shells, discard them.
When you have sorted the mussels, rinse the ones in the colander under the tap.
Chop the parsley and add it, along with the cream, to the cider sauce. Gently tip in the mussels, stir gently, then put a lid on and turn up the heat. After 2-3 minutes, remove the lid – the mussels are cooked when they have opened.
To serve, ladle into bowls and serve with bread on the side. If any mussels haven’t opened during cooking, don’t eat them.