Heat the 100ml of vegetable stock in a large saucepan and add the sliced spring onions, cook for 5-10 minutes until they begin to soften.
Add in the thinly sliced fennel and the potato and the lemon juice. Cook this mix for 5 minutes until the fennel begins to soften.
Pour in the stock and bring the soup to the boil. Turn down the heat and simmer for approximately 20 minutes or until the potatoes and fennel are tender.
Puree the soup and pass through a strainer to remove any stringy bits of fennel.
Return the soup back to the pan and season with salt and pepper.
Place the finely chopped garlic into a small bowl; add in the lemon and orange zest, finely chopped parsley and finely chopped olives and chilli. Stir to combine and set aside in the fridge until ready to use.
Serve soup garnished with a spoonful of olive gremolata.