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 10 minutes, stirring occasionally. Add cucumbers, lemon juice and stock. Bring to a boil over medium heat. Reduce heat to low and simmer for 20 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.