Melt the butter in a large pan and cook the onion and garlic over medium-low heat for 3-4 minutes until softened and lightly golden. Pour in the wine, turn up the heat, and bring to a steaming boil. Add the mussels.
Cover and cook over high heat for 4-5 minutes until all the shells have opened (give the pan a good shake halfway through the cooking time, holding the lid firmly in place.) Transfer the mussels to a serving dish, discarding any that remain closed. Leave the juices behind in the saucepan.
Add the fish stock to the juices, bring to a boil, then reduce the heat. Stir the reduced-fat creme fraiche and parsley into the pan juices, add pepper, and check for salt. Ladle the pan juices over the mussels and serve hot.