Heat oil and butter in a medium saucepan over a medium heat. Add garlic and prawn heads and sweat until opaque. Pour in stock and bring to the boil. Remove prawn heads.
Pour in tomato passata, prawns, fish and mussels and bring to the boil. Reduce heat to simmer for 3 - 5 minutes or until fish is cooked through. Season with salt and pepper to taste.
To serve, ladle soup into serving bowls, drizzle with cream and sprinkle with parsley. Serve with bread and lemon to the side.