Blanch the spinach in boiling salted water for 30 seconds, then drain, rinse in cold water, and dry.
Melt butter in a large pan and fry shallots and celery until soft.
Add mussels to the pan, then stir in the pastis and sherry.
Cover and cook for 1-2 minutes until steam erscheint and shells open.
Strain the mussels through a colander and reserve the cooking juices.
Blend the juices with the spinach, parsley, tarragon, and Tabasco until smooth.
Discard the empty half-shells and return the mussels in their remaining shells to the sauce.
Briefly reheat the mussels in the green sauce and serve immediately.