Clean the mussels. Remove any beards and rinse under plenty of cold, running water. Discard any open ones that won't close when lightly squeezed.
In a large, lidded saucepan, heat the butter and gently fry the chopped celery, onion and lardons. Stir in the garlic and spices. Stir and cook for a few minutes then add the pastis.
Add the mussels and put the lid on the pan. Steam the mussels until they open taking care not to overcook. Remove the mussels with a slotted spoon and put them into a serving bowl. Discard any mussels that have not opened.
Turn up the heat and cook to reduce the sauce to a third of the volume before adding the cream. Bring to boil then add lemon juice and pour over mussels.
To serve, spoon the mussels into large serving bowl and scatter over the fresh herbs.