Heat the oil in a large saucepan, add the onions and cook for 5–10 minutes 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. Turn down the heat, cover the pan and simmer for about 25 minutes or until the ingredients are tender.
Make the gremolata: mix together the garlic, lemon rind, chopped fennel fronds and parsley then stir the chopped olives into the herb mixture. Cover and chill.
Liquidize the soup and pass it through a sieve to remove any strings of fennel. The soup should not be too thick, so add more stock if necessary. Return it to the rinsed pan. Taste and season well with salt, pepper and plenty of lemon juice. Pour into warmed bowls and sprinkle each serving with a portion of the gremolata.