In a large saucepan, heat oil over medium heat; cook onion, garlic, ginger, cumin, coriander, turmeric and red pepper flakes, stirring until onion is softened, about 3 minutes.
Add broth, sweet potatoes and coconut milk; bring to a boil. Cover and reduce heat to low; cook for 12 to 15 minutes or until potatoes are tender.Let cool slightly.
Puree with a hand held blender or potato masher. Stir in lime juice.Season to taste with salt.