Pour the wine into a large heavy-based pan with a tight-fitting lid. Add the onions, garlic, bay leaves, thyme and peppercorns and bring to a simmer.
Now tip in the mussels and cover the pan, keeping it over a medium heat. Shake the pan now and then. Check to see that the mussels have opened after about 5 minutes and, when they are all open, tip the lot into a colander set over a bowl to catch the stock. Discard any mussels that have refused to open. Remove the flesh from some of the mussels and discard these shells.
Wipe the pan and return it to the heat. Melt the butter and add the saffron, crème fraîche, the mussel liquor, and season. Bring to the boil and simmer for a couple of minutes, then return the mussels. Add the parsley and cook for a minute to warm the mussels through, and serve immediately in warmed bowls with crusty bread.