Heat the oil in a large pot over moderate heat. Add the onion, ginger, and curry powder and cook for 10 minutes, until onions are soft but not brown. Add the potato, carrots, and chicken broth and bring to a boil. Reduce heat and cover, cooking for 30 minutes or until the vegetables are tender. Let the soup cool a little, and then puree in batches in a blender or food processor. Return to the cooking pot and adjust the seasoning with salt and pepper. Garnish with the mint chiffonade before serving.