|Warm the oil in a large dutch oven over medium heat. When hot, add the onions, garlic, leeks, celery, and watercress. Cook for 2 minutes.|
|Add the stock, potatoes, salt, and pepper. Bring to a boil, then simmer, covered, for about 30 minutes.|
|Wash and finely chop the fresh herbs.|
|Purée the soup with a hand blender.|
|Stir in lemon juice.|
|Mix in fresh herbs and garnish with lemon zest prior to serving.|
source: inspired by the delicious potage at Harrow SF
Technique adapted from Jacques Pepin's watercress soup.