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Heat the oil in a large pan, then add the spring onions, garlic, lemongrass and three-quarters of the ginger and cook for 2 mins until aromatic, then tip in the sweet potato. Give everything a good mix so the sweet potato is well coated, then add the coconut milk, stock and 500ml water. Bring to the boil, then simmer for around 25 mins until the sweet potato is cooked through.
Meanwhile, tip the remaining ginger, the chilli, sugar, three-quarters of the lime juice and most of the coriander (reserving a few leaves for a garnish) into a food processor and blitz until smooth. Transfer the sambal to a small jug and set aside.
Blitz the soup with a hand blender until smooth, then season to taste with the remaining lime juice and some salt and pepper. Divide the soup between bowls and top with the coriander sambal. Garnish with the reserved coriander leaves.