Preheat an oven to 425 degrees F. Mix sweet potatoes, onions, thyme and sage in large bowl. Drizzle with olive oil and salt, and toss to coat. Spoon mixture onto a baking sheet, spreading vegetables out in a single layer. Place baking sheet in oven and roast until vegetables are caramelized, stirring occasionally, about 25 minutes.
Remove vegetables from oven and process in a blender until pureed. Slowly add chicken broth and process until smooth. Pour soup into a large saucepan and add chiles, lime juice, sugar, nutmeg and pepper. Bring soup to a boil, reduce to a simmer and cook for 15 minutes. Remove and discard the whole chipotle chiles, stir in cream and adjust seasonings. Ladle into soup bowls and garnish with green onion, fried sweet potato chips and crema.
Make the sweet potato chips: Peel sweet potato. Using a mandoline or food processor fitted with the 2 millimeter slicing blade, cut into 1/1-6 inch slices, lengthwise.
Heat oil in a large stock pot or saucepan to deep-fry temperature (350 degrees F). Fry sweet potatoes, a handful at a time, until pale golden and crisp. Remove with a slotted spoon, drain on paper towels, and sprinkle with salt. Reserve a day or two in plastic bags.
For the crema: Whisk the cream and buttermilk together. Cover and set in a warm place (a gas oven with just the heat from the pilot is fine) for 8 hours. Crema may be kept in the refrigerator as long as a week.
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