Melt butter in a large pot over medium heat. Add onion, celery, and carrot, cooking until soft, about 5 minutes. Stir in garlic and cook for another minute.
Sprinkle flour over the vegetables and stir well. Cook for 2-3 minutes, until the flour turns a light golden brown.
Gradually whisk in seafood stock, making sure no lumps form. Stir in tomato paste, Old Bay seasoning, and cayenne pepper. Bring to a gentle simmer and cook for 10 minutes.
For a smooth bisque, use an immersion blender to puree the soup. Alternatively, blend in batches and return to the pot.
Reduce heat to low and stir in heavy cream, milk, and sherry (if using). Simmer for 5 minutes.
Gently add shrimp and crab meat, cooking for 5-7 minutes until shrimp are pink and cooked through.
Season with salt and pepper to taste. Serve hot, garnished with fresh chives or parsley.
Notes
For a dairy-free version, use olive oil instead of butter and coconut milk instead of cream.