Melt butter in a large, heavy-bottomed saucepan over medium heat. Stir in flour and cook, stirring, until roux is the color of peanut butter, about 20 minutes.
Add onion, celery, and bell pepper. Cook until onions are translucent and celery and bell pepper are tender.
Add garlic, basil, black pepper, white pepper, cayenne pepper, salt, and paprika and cook for 2 minutes.
Stir in hot sauce and stock and bring to a gentle simmer.
Add crawfish tails and green onions. Simmer for about 10 minutes, stirring occasionally.
Serve over steamed rice topped with parsley.