Boil the kettle. Heat the vegetable oil in a large saucepan and fry the onion until just soft, about 4-5 mins. Stir in the chipotle paste and cook for 1 min more.
Dissolve the stock cube in 1.2 litres boiling water from the kettle, and add to the pan along with the sweet potato. Bring to the boil, then simmer, covered, until the sweet potato is soft, about 10 mins. Whizz with a stick blender until smooth.
Ladle the soup into bowls and serve with a dollop of crème fraîche, a scattering of coriander leaves and some crusty bread rolls.