Clean the mussels by placing them in a colander or bowl under cold running water. Rinse to get rid of any debris or seaweed on their outer shells. Pick up each mussel and check that it's firmly closed, discard any that have been opened.
Chop the shallots and onion.
In a pan, melt the butter and fry the shallots and onion over low heat until golden.
Add the chopped garlic clove and the bay leaf and stir.
Pour in the white wine, season with pepper and reduce.
Add the mussels, cover and leave on a high heat for 5 minutes.
Stir from time to time to allow the mussels to open.
Remove the mussels and keep the cooking juices: filter them to remove any leftover sand.
Add the flour and cream. Mix well.
Pour the sauce over the mussel to serve!