Heat oil in large frying pan, cook sweet potato, onion and garlic, stirring for approximately 10 minutes. Add the lemon rind, rosemary, stock and water and bring to the boil.
Reduce the heat and let simmer (covered) for about 15 minutes or until the sweet potato is soft. Let cool for 15 minutes.
Blend soup, in batches, until smooth.
Return soup to a pan and add lemon juice and stir over medium heat until hot.
