Place the onion, bay leaves, parsley sprigs, peppercorns, thyme, shallots, and the white wine in a large pan, and bring to a gentle boil over medium-high heat. Let cook for about 6 minutes, until the mixture is almost dry.
While the herbs and wine are cooking, de-beard the mussels and rinse them thoroughly, then add to the wine and herb mixture. Cover and cook, shaking the pot regularly, until the mussels are open. Cook the mussels for another minute. Discard any mussels that don’t open, then transfer them to a large serving bowl and keep them warm.
Whisk the crème fraîche into the cooking juices in the pan and bring just to the boil. Remove from the heat, and strain the liquid over the mussels. Season them with a generous amount of freshly ground black pepper, and serve.