How to Make It
Heat the oil in a large pot over medium heat. Add the onion, ½ teaspoon salt, and ¼ teaspoon pepper and cook, stirring occasionally, until tender, 6 to 8 minutes.
Add the sweet potatoes, chopped apple, broth, nutmeg, and 1½ cups water. Bring to simmer and cook, covered, until the potatoes are tender, 12 to 15 minutes.
Working in batches, transfer the mixture to a blender and puree until smooth, adding more water if necessary to reach the desired consistency. (Alternatively, use a handheld immersion blender in the pot.)
Serve the soup with the blue cheese, walnuts, crackers, and sliced apples.