Add onion, celery, bell pepper, garlic and andouille and saute for about 5 minutes until vegetables are wilted.
Sprinkle in flour and stir constantly with a wire wish to make a light roux.
Add oyster liquid and heavy whipping cream, stirring to incorporate and bring to a low boil. Reduce heat and simmer until cream has slightly reduced and has a soup consistency. Add oysters, green onions and parsley. Season with salt, cayenne and hot sauce to taste.
Ladle into soup bowls and serve with a pat of butter on top.