Wash the mussels thoroughly in two or three changes of cold water. Remove the beards from the mussels and discard any shells that are broken. Then sharply tap the mussels one by one, and throw them away if they do not close. Leave all the remaining mussels to soak in cold water for 2-4 hours to get rid of any sand or grit.

Pour the wine or fish stock into a large pan. Finely chop the shallots, crush the garlic, chop the celery and add to the pan, together with the sugar and half the chopped parsley.

Simmer gently for 5 minutes. Increase the heat, add the mussels, cover the pan and cook for another 5 minutes, shaking the pan continuously until all the shells are open. Remove and discard any shells that remain closed.

Bring a large pan of water to the boil-without adding salt. Add the olive oil and spaghetti and cook, according to the instructions on the packet, leaving the pasta cooked but firm to the bite.

Meanwhile, tip all the cooked mussels into a muslin-lined colander, reserving the juices. Boil the juice for a few minutes until it has been reduced by half. Remove the mussels from their shells, keeping a few unshelled ones aside for garnish.

Drain the spaghetti and tip it back into the pan along with the reduced mussel stock; and cook gently together for 2 minutes. Then add the mussels and the other half of the parsley, and mix well and season to taste. Divide between the warmed soup bowls, garnish with the reserved mussels and serve.