Heat the oil in a large saucepan and add the garlic. Fry until the garlic just begins to colour and then add the leeks, sage and bay leaves. Saute for 5 minutes.
Add the lemon, sweet potato and chickpeas. Cover with the stock and bring to the boil. Cook until the vegetables are tender then check the seasoning.
Stir in the spring onions and the parmesan ( if using) at the last minute.