Combine first 5 ingredients in a large soup pot. Stir in broth. Bring to a boil; reduce heat, partially cover, and simmer 30 minutes or until potato and celery root are very tender.
Add butter, stirring until melted and blended into soup. Puree soup using an immersion blender (or a standard blender, working in batches). Stir in nutmeg. Top each serving with smoked trout and toasted hazelnuts. Garnish, if desired.