Heat oil in large pot. Add leeks and onion; cook over moderately low heat, stirring often, until softened - about 10 min. Sprinkle with fennel and stir 30 seconds.
Add potatoes, broth, and water. Simmer 25 min, until soft. Add arugula and cook about 10 min longer, or until stems are soft. Add salt, pepper and liquor to taste.
Puree to rough or smooth texture, as you like. Adjust seasoning. Serve hot or chilled, topped with yogurt and garnished with arugula slivers. As watercress changes potato soup into something that doesn't taste quite like any of the individual ingredients, so arugula transforms this traditional soup base.
Flexible, the soup can be pureed to a chunky or a smooth texture, and served hot, as a main dish, or chilled, as a first course.
From "Uncommon Fruits and Vegetables" by Elizabeth Schneider