1. Place onion in 2-quart microwave-safe casserole. Cover with lid; microwave on high 1 1/2 minutes or until tender.
2. Stir in soup until smooth. Stir in cheese, milk, pimento and sherry. Gently fold in chicken. Cover; microwave on high 5 minutes or until hot and bubbling, stirring once during cooking. Serve over patty shells.