Heat the oil in a large saucepan over medium-high heat. Add the leeks and cook, stirring, for 2 minutes. Add the garlic and fennel and cook 1 minute more. Add the sherry, tomatoes and their juices, salt, and red pepper and bring to a boil. Reduce heat and simmer 15 minutes. Add the fish and mussels and simmer gently until the fish is cooked through and the mussels have opened, about 5 minutes. Stir in the parsley and ladle the soup into individual bowls. Serve with the olives.
Tip: Try this dish with steamed crab in place of the fish and mussels. Ask your fishmonger to crack and clean the crab for you. Add the crab, in the shell, to the soup and simmer until the meat is heated through, about 5 minutes.