Scrub the mussels in cold water, scrape off any barnacles and pull away the dark hairy beards that protrude from the shells. Discard any with broken shells or any open mussels that do not close when tapped sharply.
Melt the butter in a large saucepan, add the onion and garlic and cook for 2–3 minutes until transparent and softened.
Increase the heat and tip in the mussels with the wine, then cover and cook for 3 minutes or until all the shells have opened. Discard any that remain closed.
Pour in the cream and heat through briefly, stirring well. Add the parsley, season well with salt and pepper and serve immediately in large bowls, with crusty bread to mop up the juices.