Wash the mussels and discard any that have broken or cracked shells or open ones that don’t close when tapped on a hard surface.
Melt the butter in a very large pan with a lid over a medium heat. Add the shallot, leek and garlic and cook for 5 minutes. Pour in the cider, bring to the boil and add the mussels.
Cover and cook for 5-7 minutes, shaking the pan occasionally, until all the mussels have opened. Discard any that are still closed after cooking.
Remove the mussels with a slotted spoon and divide among 6 warmed deep bowls. Return the pan to the heat and bring back the liquid to the boil.
Bubble for 2-3 minutes to reduce then stir in the cream and parsley. Season then pour over the mussels.