Discard any mussels that are open before cooking and any that stay closed after cooking.
Place a large pot over a high heat and brown the pancetta pieces until just golden and sizzling. Add in a knob of butter, allow it to melt and then add the onion and garlic. Cook gently for three minutes until the onion is soft.
Add in the cider and allow to bubble away for a few minutes so all the flavours mingle in together. Tumble in the mussels, cover with a lid and allow them to steam for about four minutes until they open, making sure to give the pot a good shake once or twice during the cooking time.
Remove from the heat and stir in the cream and parsley and season with sea salt and ground black pepper.
Serve with some crusty bread to mop up the liquid!