Heat the olive oil in a large saucepan and gently fry the onion over a medium heat until softened but not browned. Add the sweet potato and carrots and cook, covered, for 10 minutes or so, until the vegetables are soft but still with some bite.
Pour in the stock, bring to the boil and then simmer, covered, for 30 minutes until the vegetables are tender.
Pour into a blender and whizz until smooth - do this in batches if you need to. Return to the pan and gently reheat.
Season if desired and either stir in the chilli oil or serve garnished with a few drops of the oil.