Melt the butter in a large saucepan over medium-low heat. Add the green onions and celery. Sauté until tender.
Add the flour into the butter and vegetables little by little, blend it until incorporated. Continue cooking and stirring for about 2 minutes.
Slowly stir in the warmed milk and continue cooking until thickened.
Add the freshly ground black pepper, tomato paste, and heavy cream.
Stir in the crab, shrimp and the sherry. Bring to a simmer.
Serve it hot when you see it has a creamy consistency.