Gather the ingredients.
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, about 2 minutes.
Blend the flour into the butter and vegetables until well incorporated. Continue cooking, stirring, for about 2 minutes.
Warm the milk in another saucepan over medium heat.
Slowly stir the warmed milk into the butter and vegetables. Continue cooking, stirring until thickened, about 3 minutes.
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.
Stir in the shrimp, crab, and sherry. Bring it to a simmer.
Serve hot, garnished with parsley, cilantro, and/or chopped green onion tops.
