Melt the butter in a Dutch oven or large saucepan, over medium-low heat; add the chopped green onion and celery.
Sauté, stirring, until tender.
Blend the flour into the butter and vegetables until well incorporated.
Continue cooking, stirring, for about 2 minutes.
In another saucepan, over medium heat, warm the milk.
Slowly stir in the warmed milk and continue cooking and stirring until thickened.
Add the freshly ground black pepper, tomato paste, and heavy cream.
If desired, puree the soup in a blender or food processor at this point and then return it to the saucepan; if available, an immersion blender is best to accomplish this task.
Stir in the crab, shrimp, and the sherry; bring to a simmer.
Serve hot.
