In a 4-quart Dutch oven heat butter and oil over medium heat. Add fennel and the two leeks; cook about 10 minutes or until crisp-tender, stirring occasionally.
Carefully add sherry to Dutch oven. Cook and stir about 2 minutes or until sherry is nearly evaporated. Carefully add broth and potatoes. Bring to boiling; reduce heat. Simmer, covered, about 30 minutes or until vegetables are very tender. Cool slightly.
Transfer fennel mixture in batches to a blender or food processor. Cover and blend or process until smooth. Return pureed mixture to Dutch oven. Stir in thyme; heat through. Season to taste with salt.
Ladle soup into bowls. Sprinkle with pepper and, if desired, garnish with additional sauteed leeks.