On Saturday night, El-Khoury scored a triumph in the title role, combining an exquisite delicacy of sound with affecting vulnerability — yet producing plenty of steel in the confrontation scene in which she denounces Elizabeth as a “vil bastarda.” On Sunday, few in the house realized that El-Khoury had just sung this taxing role the previous evening — and you couldn’t tell it from her Sunday performance, either. Skillfully saving her voice when she could, singing full-out when it counted, El-Khoury rallied for a remarkable show and a final mighty effort, a brave high D. Brava!