Sort and wash beans; place in a large mixing bowl. Cover with water 2 inches above beans; let soak overnight. Drain well.
Sauté celery and onion in 2 tablespoons butter in a large Dutch oven until tender. Add water, ham bone, and beans; stir well. Bring to a boil. Reduce heat; cover and simmer 3 hours or until beans are tender. Remove and discard ham bone.
Drain bean mixture, reserving cooking liquid in Dutch oven. Puree beans, and return to Dutch oven; stir well. Bring to a boil; stir in remaining butter, salt, pepper, and mustard. Reduce heat; cover and simmer an additional 15 minutes. Stir in sherry.
Ladle soup into individual bowls; garnish each serving with a lemon slice. Serve warm.