Melt the butter in a Dutch oven or large saucepan over medium-low heat; add the chopped green onion and celery. Saute, stirring, until tender.
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 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.
Stir in the crab, shrimp, and the sherry. Bring to a simmer.
Tips and Variations
Feel free to add small cooked bay scallops or lobster instead of the shrimp.
Replace the green onions with finely chopped shallots.
If desired, garnish the soup with thinly sliced green onion tops, cilantro, or parsley. Or add a spoonful of sour cream and swirl it.