Cook the onion in a little oil until soft. Add the wine and mussels, cover and shake occasionally until the mussels are open. Now add some pepper from the mill, then the cream. Bring to the boil, shaking again, and transfer to a tureen. Sprinkle with parsley and serve immediately. DISCARD ANY MUSSELS THAT DID NOT OPEN. Do not use salt as the mussels will be naturally salty.