Creamy Carrot and Sweet Potato Soup  3 tablespoons butter, divided   1 cup chopped onion   1/4 teaspoon ground cinnamon   1/4 teaspoon ground nutmeg   4 3/4 cups cubed peeled sweet potatoes (about 1 1/2 pounds)   3 1/2 cups water  3 cups fat-free, less-sodium chicken broth   3 cups chopped carrot (about 1 pound)   1/4 cup half-and-half   1/2 teaspoon salt   1/4 teaspoon freshly ground black pepper   1/3 cup reduced-fat sour cream   2 tablespoons chopped fresh flat-leaf parsley  Preparation…