Melt the butter in a small saucepan over a low heat and gently cook the shallots, garlic, celery and bay leaf until soft but not brown. Add the pastis and continue cooking until the liquid has evaporated.
Stir in the flour and gently cook for a minute or so then slowly add the milk a little at a time, stirring continuously, to make a smooth, thick sauce.
Meanwhile, tip the mussels and wine into a separate pan, cover with a lid and steam the mussels for 3-4 minutes until the shells have just opened.
Place a colander over the pan containing the white sauce and drain the mussel liquor into the sauce. Return the mussels to their pan and cover with a lid to keep warm.
Working quickly, whisk the mussel liquor into the sauce, bring it to the boil and continue boiling until it is the consistency of double cream. Stir in the cream, pour the sauce into the pot of mussels and serve immediately with crusty bread.