Soak the mussels in a sink of cold water. Pull off the beards, scrape off any large barnacles and discard any mussels that are damaged or do not close when sharply tapped. Wash them in a second change of water and drain.
Place a large pan over a high heat. Put the mussels in, cover, and steam them for 5-7 minutes, shaking or stirring after a few minutes. Meanwhile, put the cider, shallots and bay leaf in a small saucepan and boil to reduce the liquor by half. Strain the mussels and decant their liquor into the cider pan, leaving behind the last little bit, which will be gritty. Return the mussels to their pan and set aside, covering with a lid to keep them warm.
Simmer the cider sauce vigorously for a few minutes until reduced by a third, then stir in the crème fraîche and return to a simmer. Season the sauce with black pepper and pour it over the mussels. If the shellfish have cooled down, return them to the stove for a minute or two. Serve in warmed shallow bowls with lots of bread.