In food processor or with knife, finely chop onion, celery, carrot, and garlic.
In a large pot or Dutch oven, heat the oil over medium heat. Add the onions, celery, carrot and garlic and cook, stirring until the onion softens but does not brown, about 8 minutes.
Stir in the thyme, and cook for 30 seconds. Add the sweet potatoes, broth and curry powder and bring to a boil over medium high heat. Lower heat to medium and simmer until the sweet potatoes are tender, about 20 minutes. Purée until smooth and season with cayenne, salt and 1/2 teaspoon pepper.
In a small bowl, whisk together the sour cream, lemon juice, and 1/4 teaspoon pepper. Ladle soup into bowls and spoon a dollop of the sour cream mixture on top of each.