Melt the butter in a large pan and cook the onion and garlic over a medium low heat for 3-4 minutes until softened and lightly golden. Then pour in the wine, turn up the heat and bring to a steaming boil. Now add the mussels.
Cover and cook over a high heat for 4-5 minutes until all the shells have opened - give the pan a good shake half way through the cooking time, holding the lid firmly in place, discard any that remain closed.
Remove the mussels to a serving dish, and leave the juices behind in the pan. (If you suspect any sand or grit from the mussels might be in the juices, strain through a clean cloth or muslin).
Add the fish stock to the juices, bring to the boil, then reduce the heat. Stir the creme fraiche and the parsley into the pan juices, add pepper and check for salt.
Ladle the pan juices over the mussels and serve hot.