Step #1 Melt the butter In a large-ish pan & cook the onion & garlic over a medium low heat for 3-4 mins until softened & lightly golden.
Step #2 Then pour in the wine, turn up the heat & bring to a steaming boil.
Step #3 Now add the mussels.
Step #4 Cover up & cook over a high heat for 4-5 mins 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.
Step #5 Remove the mussels to a serving dish, & leave the juices behind in the pan.
Step #6 (If you suspect any sand or grit from the mussels might be in the juices, strain through a clean cloth or muslin).
Step #7 Add the fish stock to the juices, bring to the boil, then reduce the heat.
Step #8 Stir the creme fraiche & the parsley into the pan juices, add pepper & check for salt.
Step #9 Ladle the pan juices over the mussels & serve hot.