This recipe can be prepared in 45 minutes or less.
The fennel is the special flavor in this soup.
Melt butter in heavy large pot over medium-high heat. Add leeks and fennel and sauté until leeks are translucent, about 7 minutes. Add broth and potatoes and bring to boil. Reduce heat to medium-low. Simmer soup until potatoes are very tender, about 25 minutes. Working in batches, purée soup in blender. Return to same pot. Rewarm soup if necessary. Season with salt and pepper. Ladle soup into bowls; garnish with reserved fennel fronds and serve.