Heat the oil in a large saucepan, add the spring onions and cook for 5 minutes or until beginning to soften. Add the fennel, potato and lemon rind and cook for 5 minutes until the fennel begins to soften. Pour in the stock and bring to the boil. Reduce the heat, cover and simmer for about 15 minutes or until the ingredients are tender.
Meanwhile, make the gremolata. Mix together the garlic, lemon rind and parsley, then stir the chopped olives into the mixture. Cover and chill.
Liquidize the soup in a food processor or blender and pass it through a sieve. The soup should not be too thick, so add more stock if necessary. Return it to the rinsed pan and warm through. Taste and season with pepper and plenty of lemon juice. Pour into warm bowls and sprinkle each serving with gremolata, to be stirred in before eating. Serve with slices of toasted crusty bread, if liked.