2 tablespoons unsalted butter (1/4 stick)
3/4 cup finely chopped yellow onion (about 1/2 medium onion)
2 1/4 teaspoons grated fresh ginger (from about a 2-inch piece)
4 star anise pods (or 2 teaspoons star anise pieces)
2 1/2 pounds sweet potatoes, peeled and coarsely chopped
1 quart (4 cups) low-sodium chicken or vegetable broth
1 cup water
1/2 cup heavy cream or half-and-half (optional)
6 tablespoons sour cream or crème fraîche