Separate the leaves from the celery stalks, wash and pat dry, then set the leaves aside. Roughly chop the stalks.
Heat the butter in a medium saucepan. Add the onion and celery, cover and cook gently for 3-4 minutes to soften, stirring occasionally. Add the stock, season lightly and bring to a simmer. Cover and simmer for 30 minutes. Now add all but a handful of the celery leaves and si