Heat butter and oil in wide skillet set over medium-high heat. Add prawns and scallops and cook until just cooked through. Lift prawns and scallops out and reserve. Add onion and garlic to skillet, and a little more oil if needed, and cook 2-3 minutes. Stir in flour until well combined. Whisk in sherry and cook, stirring constantly until sauce thickens. Whisk in remaining ingredients, except parsley, and simmer until sauce lightly thickens. Mix in reserved seafood and heat through. Adjust seasoning, sprinkle with parsley, and serve.