Gather the ingredients.

Melt 3 tablespoons unsalted butter in a Dutch oven or large saucepan over medium-low heat; add 2 tablespoons chopped green onion and 2 tablespoons chopped celery. Sauté, stirring, until tender, about 2 minutes.

Stir 3 tablespoons all-purpose flour into the butter and vegetables until well incorporated. Continue cooking, stirring, for about 2 minutes.

Warm 2 ½ cups 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 ½ teaspoon freshly ground black pepper, 1 tablespoon tomato paste, and 1 cup 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 8 ounces cooked shrimp, 8 ounces crabmeat, and 2 tablespoons sherry wine. Bring it to a simmer.

Serve hot, garnished with 3 tablespoons parsley, cilantro, and/or chopped green onion tops.