Melt the butter in a dutch oven or large saucepan over medium-low heat; add the chopped onion and celery. Sauté, stirring, until tender, about 2 minutes. Add in garlic.
Blend the flour into the butter and vegetables until well incorporated. Continue cooking, stirring, for about 2 minutes. Add in seasonings and tomato paste.
Slowly stir the milk into the butter and vegetables. Continue cooking, stirring until thickened, about 3 minutes. Add in the 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 cooked shrimp, shredded crab, corn kernels and sherry. Bring it to a simmer. Serve hot, garnished with parsley and additional chopped celery and corn.
