In large saucepot over medium high heat, heat oil. Add carrot, onion, leek and garlic and cook, stirring occasionally, until just tender, about 5 minutes. Add potatoes and chicken stock and simmer until potatoes are tender, about 15 minutes. Add coconut milk, cilantro, brown sugar, red curry paste and fish sauce and simmer 5 minutes. Puree with immersion blender or in batches in large blender until smooth.