Melt butter in a pot over medium heat. Add chopped onion and cook 3–4 minutes until soft.
Stir in garlic and cook about 20 seconds until fragrant.
Sprinkle in flour and stir 1–2 minutes until smooth.
Stir in tomato paste and cook 30 seconds. Slowly whisk in seafood stock until the mixture becomes smooth.
Bring to a gentle simmer for 8–10 minutes.
Lower heat and stir in heavy cream and half-and-half. Keep the heat low so it stays smooth.
Add Old Bay, salt, black pepper, and cayenne if using.
Add shrimp and cook 2–3 minutes until pink. Gently fold in crab and warm 1–2 minutes.
Stir in parsley and serve hot with bread for dipping.
