Heat the oil in a large soup pot over medium heat. Add the sliced leeks and sauté until tender, about 5 minutes. Stir in the ginger and cook until fragrant, 1 minute. Add the sweet potatoes, potatoes, apples, and water, and bring to a simmer. Sprinkle in salt to taste, reduce heat to a simmer. Cover and simmer for 45 minutes to an hour until the sweet potatoes and potatoes are tender.
Purée the soup using a blender (work in batches and place a kitchen towel on top to avoid splashing) or use an immersion blender. Purée until very smooth. Return to pot to allow to heat through. Adjust the salt if needed. Serve with a bit of lime juice and chili powder spinkled onto each bowlful.