Sauté vegetables: In a large pot or Dutch oven, melt the butter over medium heat. Add the chopped onion, diced carrots, and diced celery. Cook for 5-7 minutes, stirring occasionally, until softened.
Add garlic and flour: Stir in the minced garlic and cook for 1 minute more, until fragrant. Sprinkle in the flour and cook for 1 minute, stirring constantly, to make a roux.
Whisk in broth and cream: Gradually whisk in the seafood broth (or chicken broth), heavy cream, and sherry (or white wine) until smooth. Bring to a simmer, then reduce heat to low and cook for 15 minutes, stirring occasionally, until slightly thickened.
Season: Stir in the Old Bay seasoning and black pepper. Season with salt to taste.
Add seafood: Add the cooked shrimp and crab meat. Cook for 5 minutes more, or until heated through. Be careful not to overcook the seafood, as it can become rubbery.
Serve: Serve hot, garnished with chopped fresh parsley, chives, or a drizzle of cream, if desired.