In a large saucepan, melt butter. Sauté onion, celery and potatoes. Blend in flour, salt, pepper, and cook for 1 minute. Gradually add broth, milk, cream and sherry, stirring constantly. Cook on medium heat until vegetables are tender.
Reduce heat to low, add cheese and stir until melted. Add fish and cook until done about 10 minutes. Serve topped with parsley.