Rinse the mussels well in cold water, taking off any dirt, barnacles and hairy bits. Discard any that are broken or open that don't close when you press them.

Finely slice the shallot/onion and garlic. Warm the oil in a pan big enough to allow the mussels to expand and add the onion and garlic. Stir and cook for a minute or two to soften slightly. Add the wine then the mussels, cover with a lid and allow the mussels to steam. They should only take a couple minutes to open up. Give the pan a little shiggle if you need to let the mussels have more space to open.

Once all of the mussels have opened, remove them with a slotted spoon to a serving plate. Add the parsley and butter to the wine, continue to cook a minute so the butter melts then pour the sauce over the mussels and serve immediately.