If store bought Creme Fraiche can't be found you can make your own by combining 1/4 cup whipping cream and 1/4 cup sour cream. Cover and let warm to room temperature for 2 to 5 hours until mixture thickens then refrigerate for at least 2 hours.
Cook leeks and onion in hot margarine or butter in a large saucepan over medium heat until tender. Stir in flour. Carefully add chicken broth all at once. Cook and stir over medium heat until slightly thickened and bubbly. Stir in pumpkin, vanilla bean (if using), salt, pepper, and the 1/4 teaspoon nutmeg. Return to boiling. Reduce heat, cover, and simmer for 20 minutes. Remove from heat. Cool slightly. Remove vanilla bean; discard. Add pear halves and pear liquid to pan.
Process mixture in three or four batches in a food processor bowl or blender container until smooth. Return mixture to saucepan. Stir in light cream and vanilla extract, if using. Heat through, but do not boil.
To serve, ladle soup into bowls. Top with a spoonful of Creme Fraiche; sprinkle with additional nutmeg, if desired.