Melt butter in a soup pot over medium heat. Cook onion and celery for 8 minutes until softened, then stir in garlic for 30 seconds.
Add sherry and simmer for 2 minutes. Add chestnuts, broth, salt and white pepper and bring to a gentle simmer.
Cover partly and cook for 20 minutes until chestnuts crush easily against the side of the pot. Remove from the heat and blend with an immersion blender until smooth.
Stir in cream and lemon juice. Warm gently over low heat for 3 minutes without boiling and divide among 6 bowls.