1. Arrange an oven rack at center position and preheat oven to 400F.

2. Rinse and dry the pumpkin. Halve the pumpkin through the stem, and scoop out and discard the seeds and strings. Cut each half into large wedges, and then cut the wedges into 3- to 4-inch pieces. Place the pumpkin pieces, cut sides down, on a large, heavy rimmed baking sheet. Add enough water to the pan to measure 1/4 inch up sides. Cover tightly with aluminum foil.

3. Bake until pumpkin is tender when pierced with a sharp knife, for 45 to 50 minutes. Remove from oven and let cool.

4. When pumpkin is cool enough to handle comfortably, use a small, sharp knife to remove peel. Cut the pieces into 1-inch cubes to yield 8 cups. (Save any remaining pumpkin for another use.)

5. Heat butter in a large, heavy pan (with a lid) over medium heat. When hot, add leeks and cook, stirring, until they are softened and translucent, for about 3 minutes. Add the pumpkin cubes, stock, rosemary, 2 teaspoons salt, and cayenne pepper. Stir to combine and bring mixture to a simmer. Reduce heat, cover, and cook for 10 minutes to meld flavors.

6. Purée the soup in batches in a food processor, blender or food mill, and return soup to the pot. (Or use an immersion blender to purée the soup in the pot.) Whisk in 1 cup of the crème fraîche. Taste soup and season with salt, as needed. (Soup can be prepared 2 days ahead. Cool, cover, and refrigerate. Reheat over low heat.)
 
7. To serve, ladle soup into 6 soup bowls. Garnish each serving with a dollop of the remaining crème fraîche and sprinkle with chopped walnuts. If desired, garnish each serving with a rosemary sprig.

Betty Rosbottom, Sunday Soup, Chronicle Books (2008.)