Wash spinach carefully and discard any thick stalks.
Melt butter over medium heat. Add onion and celery and saute until onion is translucent (about 3 minutes). Stir in flour, add salt and pepper and cook for 2 minutes.
Add chicken stock, stirring well. Heat until it starts to boil, stirring frequently. Add spinach and simmer for 15 minutes. Remove from heat and allow to cool for 15 minutes.
Using a food processor with steel blades, or a blender, puree the mixture in batches until it is smooth with dark green flecks.
Return to a medium heat, taste and correct seasoning. Stir in cream, sherry and nutmeg. Do not allow to boil. Serve immediately.
NOTES: Easy cream of spinach soup. Serves 4-6.