Heat oil in a large soup pot over medium heat. Add onions and ginger, and cook until softened, about 3-4 minutes. Add carrots and sweet potatoes. Cook until they begin to soften, about 3-4 minutes.
Add curry powder, mix well, and cook 1 minute to develop flavor. Add kaffir lime leaves, lemongrass, vegetable stock and salt. Bring to a boil, reduce heat to low, and cook, covered, for 20-25 minutes, until vegetables are soft.
Stir in coconut milk and cook 2 more minutes. Remove kaffir lime leaves.
Puree soup in batches in a blender (I fill the container just below the halfway mark), being careful to remove the stopper in the cover to prevent hot soup from splashing. You can also puree the soup in a food processor.
Serve immediately or refrigerate until ready to serve. This soup will keep for several days in the refrigerator.