Slice the shallot into half rings, and cut the fennel and celery into small pieces.
Heat butter in a pan and add the chopped vegetables, onion, and garlic.
Cook everything until it's soft, all the while stirring well!
Pour in the pastis and stir it well into the vegetables.
Add your mussels to the pan, cover with the lid, and give it a shake.
When the mussels start to open slightly but are not quite cooked yet, add the cream.
Give it one more shake and let the pot simmer until all mussels are fully open.
Sprinkle the parsley over the pot and enjoy!