Wash and clean the mussels.
Discard any that are open and do not close when tapped sharply.
Take a large lidded pan that will hold all the mussels.
Add the mussels, butter, onion, white wine and half the parsley.
Put over a high heat.
Cover and cook until they are all open, removing the lid to turn the mussels occasionally.
When they are all open remove from the heat and set aside for about 30 minutes to let any grit settle to the bottom of the pan.
Scoop out the mussels with a large slotted spoon and divide between four plates.
Pour all the juices from the pan over the mussels, holding back the last tbsp or so.
Sprinkle the remaining parsley over the mussels and serve with mayonnaise. and chips.