To preserve the brilliant green color of this soup, serve it chilled; it's a perfect beginning for a warm spring dinner. Because there are so few ingredients in this recipe, use the best-quality chicken stock and olive oil you can find.
Place the olive oil in a large stockpot and heat over medium heat. When the oil is hot, add the leeks, and cook until they are soft and tender, stirring frequently, 10 to 15 minutes. Don't allow the leeks to color. When they're soft, add the garlic and fava beans; cook for 2 minutes, taking care not to burn the garlic. Add 1 cup of the chicken stock to the pan and cook until the fava beans are just tender, 5 to 8 minutes. Don't overcook the favas or they'll lose their bright color.
Transfer the soup to a blender or food processor and purée until smooth, gradually adding the remainder of the chicken stock. Strain the soup through a fine mesh sieve into a clean bowl, discarding the solids. Place the bowl in an ice bath to chill the soup quickly and preserve its bright color. If the soup is too thick, add more chicken stock. Season to taste with salt and pepper.
Serve the soup chilled, drizzled with olive and/or truffle oil and garnished with a sprinkling of fresh herbs.