Melt butter in a large pot over medium heat. Add onion and cook until softened, about 5 minutes. Add garlic and cook for 1 minute more.
Stir in tomato paste and Old Bay seasoning. Cook for 2 minutes, stirring constantly.
Pour in seafood stock and sherry (if using). Bring to a simmer.
Add shrimp and cook until pink and opaque, about 3-5 minutes. Remove shrimp from pot and set aside.
Using an immersion blender, blend the soup until smooth. Alternatively, carefully transfer the soup to a regular blender and blend until smooth. Return to pot.
Stir in heavy cream and crab meat. Heat through gently. Do not boil.
Return the cooked shrimp to the pot. Season with salt and pepper to taste. Serve hot.
Notes
Garnish with fresh parsley or chives before serving.