In large pot, combine leeks with butter and olive oil. Cook over medium heat for 5 minutes. Add fennel, potatoes and broth. Season with salt and pepper. Bring to a simmmer and cook for 1 hour. Stain vegetables and process until smooth in a food processor. Return vegetable puree and broth to the pot and reheat gently. Correct the seasonings, ladle into soup bowls and drizzle with olive oil.