Wash and chop the spring onions. Cut the fennel into thin slices. Peel and finely dice the potatoes. Wash the lemon with hot water and rub the peel. Squeeze out the juice.
Heat the oil in the saucepan, fry the spring onions in it until translucent, add the fennel, potatoes and lemon peel and fry for 3 minutes. Pour in the broth and bring to a boil. Cover and simmer on medium heat for another 15 minutes.
In the meantime, for the gremolata, wash the lemon with hot water and rub the peel off. Finely chop the olives. Mix the garlic, lemon peel, parsley and olives well. Put in a cool place.
Now puree the soup finely and pass through a fine sieve. The soup shouldn`t be too thick. If necessary, add a little more broth. Season with pepper and plenty of lemon juice.
Spread the soup on preheated plates and place the gremolata on top. Stir in before consumption.