Melt 2 tbs. butter in heavy large pot over medium-high heat. Add 2 c. sliced leeks (or fresh onions) and 2c. sliced fennel bulb and sauté until leeks are translucent, about 7 minutes. Add 8 c. chicken broth and 2# diced 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 fronds and serve.