Heat the oil in a heavy soup pot or Dutch oven over medium heat. Add the shallots. Cook, stirring, until tender, three to five minutes. Add carrots. Cook, stirring often, for five minutes until they begin to soften. Add the sweet potatoes, water or stock, tarragon sprigs, and orange zest, and bring to a simmer. Add salt to taste, reduce the heat, cover and simmer 45 minutes or until all of the ingredients are thoroughly tender. Remove the orange zest, and discard.

Purée the soup in batches in a blender, removing the stopper from the top and holding a kitchen towel tightly over the top to avoid splashing, or in a food processor fitted with the steel blade. Return to the pot, heat through, and adjust salt and pepper to taste. Stir in the mint or tarragon, and serve. This will keep for three to four days in the refrigerator.