2 tbsp (1/4 stick) butter
150g thinly sliced leeks (white and pale green parts only; about 3 medium)
200g potato, diced
750ml chicken broth (see recipe)
225g coarsely chopped watercress with some of stems plus small sprigs (for garnish)
225g baby spinach leaves
1 1/2 tsp fresh lemon juice
2 hard-boiled eggs, peeled, chopped (for garnish)
Melt butter in heavy large saucepan over medium heat. Add leeks and potato; sauté until leeks are slightly softened but not brown, stirring often, about 5 minutes. Add 3 cups broth; bring to boil. Reduce heat, cover, and simmer until vegetables are very tender, 10 to 12 minutes. Add watercress and spinach; stir until wilted, about 1 minute. Cool slightly.
Working with 1 cup at a time, puree soup in blender until smooth. Transfer soup to large bowl. Stir in lemon juice; season with salt and pepper. Chill until cold, about 2 hours. Thin with more broth, if desired. Ladle soup into bowls. Garnish center of each with hard-boiled eggs and small watercress sprigs.