Make the soup: In a kettle cook the onion, leek, garlic, and carrots with the bay leaf and salt and pepper to taste in the butter over moderate heat, stirring, until the vegetables are softened. Add the sweet potatoes, peeled, halved lengthwise, and sliced thin, the potato, peeled, halved lengthwise, and sliced thin, the 5 cups broth, wine, and water, simmer the mixture, covered, for 15 to 20 minutes, or until the potatoes are very tender, and discard the bay leaf.
In a blender puree the mixture in batches until it is very smooth, transferring it as it is pureed to a large saucepan, add the additional broth to thin the soup to the desired consistency, and season the soup with salt and pepper. The soup may be made 1 day in advance, kept covered and chilled, and reheated.
Make the buttered pecans: In a skillet cook the pecans in the butter with salt, to taste, over moderate heat, stirring occasionally, for 10 minutes, or until they are golden brown, and transfer them to paper towels to drain. The pecans may be made 2 days in advance and kept in an airtight container or a resealable plastic bag.
Divide the soup among bowls and top each serving with a dollop of the creme fraiche and some of the buttered pecans.
Recipe courtesy of Gourmet Magazine