HEAT the oil in a large saucepan over a medium heat. ADD the onion and cook for 5 minutes, stirring occasionally. ADD the garlic and red curry paste and cook for 1 minute, stirring constantly. ADD the sweet potato, red lentils, stock, coconut milk, fish sauce and brown sugar and bring to the boil. REDUCE the heat to medium and simmer, covered, for 20 minutes. REMOVE from the heat and allow to cool a little. PUREE the soup in two batches in a blender or food processor until smooth. RETURN to the saucepan and stir in the lemon juice. SERVE in bowls garnished with cilantro.
Variations: Replace the sweet potato with butternut squash (pumpkin); substitute the lemon juice with lime juice; and use yellow split peas rather than red lentils (in which case you'll need to cook for around 35 minutes).
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