Melt butter in a saucepan and cook shallots, garlic, celery, and bay leaf until soft.
Add pastis and cook until the liquid has evaporated.
Stir in flour for one minute, then slowly whisk in 150ml of milk to make a smooth sauce.
Steam mussels and 300ml of white wine in a covered pan for 3-4 minutes until shells open.
Drain the mussel cooking liquor through a colander directly into the white sauce.
Keep mussels warm in their pan while boiling the sauce until it reaches cream consistency.
Stir in 200ml of cream, pour the sauce over the mussels, and serve with bread.