Melt butter in a large saucepan over medium heat. Add celery, shallots, potato, and a pinch of salt. Partially cover; cook, stirring often, until vegetables are soft but not browned, about 15 minutes.
Add stock; bring to a simmer. Reduce heat to low; cover, and cook 30 minutes. Stir in nutmeg. Let cool.
Puree soup in a blender or food processor in batches until completely smooth. Pour through a fine sieve into a large bowl, pressing with the back of a spoon to extract as much liquid as possible. Return soup to pan; stir, and season with salt. Just before serving, heat soup over medium heat (do not boil). Stir in cream, and serve topped with baguette slices; garnish with celery leaves. Soup can be refrigerated, covered, up to 2 days.