1. In a medium saucepan, heat 1 tablespoon oil over medium. Add leeks and season with salt and pepper. Cook, stirring occasionally, until tender, 4 to 6 minutes. Add broth, potato, and 4 cups water. Bring to a boil; reduce to a simmer and cook, partially covered, until potato is tender, 10 to 12 minutes.

2. Add 2 bunches watercress and cook until bright green and tender, about 3 minutes. Using a slotted spoon, transfer vegetables and a little liquid to a blender; puree until smooth and pour into a bowl. Puree remaining liquid in blender and return to pot. Stir in pureed vegetables and season soup with salt.

3. In a small bowl, whisk together 1 tablespoon oil and lemon juice and season with salt and pepper. Add remaining watercress and toss. Serve soup topped with watercress.