Combine the stock, sweet potatoes, celery, onion, bay leaf, and tarragon in a large pot and bring to a boil over high heat.
Reduce the heat and simmer until the vegetables are tender, 20 to 30 minutes.
Discard the bay leaf, puree the soup in batches in an electric blender or food processor, and return to the pot. Add the half-and-half, salt, and pepper and bring to a simmer.
Garnish with a dollop of sour cream and a sprinkle of chopped herbs if desired. Don't forget the freshly ground black pepper, and some goat cheese on top might also be a nice contrast to the sweetness of the sweet potatoes. Enjoy!