In a small bowl, mix the flour with 40gr of butter and put it aside for later.
In a large saucepan, put the rest of the butter and cook the shallots a little bit over medium heat before adding the celery. Let it cook for 3-4min then pour the wine. Bring the mixture to the boil.
When it starts to boil, add the mussels and cover them. Cook them over hight heat for about 5min until they open. The unopened mussels can be junked.
Remove the mussels from the broth and leave them in soup bowls. Progressively add the small bowl of butter and flour to the broth while beating. Pour the sauce on the mussels and serve them.