Heat the oil in a large pan and add the garlic. Fry until it just begins to colour then add the leeks, sage and bay leaves.
Saute for 5 minutes before adding the lemon, sweet potato and chickpeas. Cover with stock and bring to the boil- the final thickness of the soup will depend on how much stock you add.
Cook until the vegetables are tender and check seasoning. At the last minute stir in the spring onions and parmesan.