Preparation Heat oil in heavy large pot over medium-high heat. Add fennel bulbs, onions, and potatoes; sprinkle with salt and pepper. Sauté until slightly softened, about 8 minutes. Add 4 cups broth and 2 teaspoons lemon juice; bring to boil. Reduce heat, cover, and simmer until vegetables are very tender, about 30 minutes. Cool slightly. Meanwhile, bring cream, fennel seeds, and tarragon to boil in heavy small saucepan. Reduce heat and simmer 5 minutes. Remove from heat; cover and let stand 20 minutes so that flavors blend. Working in batches, puree soup in blender until smooth. Return soup to same pot. Stir in cream mixture. Simmer until flavors blend, thinning with more broth if desired, about 5 minutes. Season to taste with salt and pepper and more lemon juice, if desired. (Can be made 1 day ahead. Cool slightly. Chill until cold, then cover and keep chilled. Rewarm over medium heat, stirring occasionally.) Ladle soup into 8 bowls. Garnish with reserved fennel fronds and serve.
Preparation Heat oil in heavy large pot over medium-high heat. Add fennel bulbs, onions, and potatoes; sprinkle with salt and pepper. Sauté until slightly softened, about 8 minutes. Add 4 cups broth and 2 teaspoons lemon juice; bring to boil. Reduce heat, cover, and simmer until vegetables are very tender, about 30 minutes. Cool slightly. Meanwhile, bring cream, fennel seeds, and tarragon to boil in heavy small saucepan. Reduce heat and simmer 5 minutes. Remove from heat; cover and let stand 20 minutes so that flavors blend. Working in batches, puree soup in blender until smooth. Return soup to same pot. Stir in cream mixture. Simmer until flavors blend, thinning with more broth if desired, about 5 minutes. Season to taste with salt and pepper and more lemon juice, if desired. (Can be made 1 day ahead. Cool slightly. Chill until cold, then cover and keep chilled. Rewarm over medium heat, stirring occasionally.) Ladle soup into 8 bowls. Garnish with reserved fennel fronds and serve.