To prepare the soup:
In a medium-size saucepan, heat oil over low heat.
Add leeks and celery and cook until vegetables are soft, about
1 minutes, stirring occasionally.
Add cucumbers, lemon juice and stock.
Bring to a boil over medium heat. Reduce heat to low and simmer for 2 minutes, or until cucumbers are tender.
Cool slightly.
To prepare Pistou:
With a mortar and pestle, mash together garlic, basil and walnuts to a paste. (you can also use a bowl with a wooden spoon).
Work in a little cheese and oil and one-third of the tomato.
Keep adding cheese, oil and tomato.
The mixture will not be a true emulsion but a barely fluid paste.
Transfer cucumber mixture to a food processor or blender and
puree, in batches, until smooth.
Ladle soup into serving bowl. Pass the pistou.
Allow 1 tablespoon of pistou per serving to be placed in the
center of the bowl, then swirl into soup.
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