Coat a pan with cooking spray, heat and add the onion and garlic. Cook until the onion is softened. Stir in the sweet potato. Cook, stirring, for 1 minute. Pour in the water, stock cubes and sugar. Bring to the boil, lower hear, cover and cook for about 20 minutes or until sweet potato is tender. Cool slightly.
Puree the mixture and return to the pan. Reheat and serve sprinkled with the parsley.