In a medium heavy bottom sauce pan heat olive oil, add chopped onion and celery and cook until onions are translucent
Add oysters and liqueur to saucepan and cook for 5 minutes
Reduce heat slightly and add half and half, stir to combine
Gently cook stew for 20 minutes
Add salt and pepper to taste
Ladle soup into bowls, drizzle a small amount of sherry over each bowl of stew and garnish with parsley, a dollop of butter
Serve with oyster crackers