Melt the butter in a Dutch oven or large saucepan over medium-low heat. Add the chopped green onion and celery, and sauté until tender.
Blend the flour into the butter and vegetables until well incorporated. Continue cooking and 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 sherry. Bring to a simmer.
Serve hot, and garnish with thinly sliced green onion tops, cilantro, or parsley, or a spoonful of sour cream swirled on top.
Tips and Variations:Swap out the shrimp for small cooked bay scallops or lobster for a variation on the seafood flavors.
Substitute finely chopped shallots for the green onions for a milder onion flavor.
Get creative with your garnishes – add your own touch with your favorite fresh herbs or spices.