Toss 1/2 cup Parmesan cheese with pasta, and keep warm.
Melt butter in a large saucepan over medium heat; add leek and celery, and saute until tender. Add pumpkin; saute 2 to 3 minutes. Add broth; bring to a boil. Cover, reduce heat, and simmer 20 minutes. Add salt, pepper, and nutmeg.
Process pumpkin mixture in a blender until smooth. Return to saucepan; stir in whipping cream, and cook over low heat, stirring often, until heated. Serve over pasta; top with remaining cheese.