Sorrel sauce is a classic French cream sauce that uses the tart, lemony potherb commonly known as sorrel (Rumex acetosa) as the defining ingredient. Oxalic acid, a naturally occurring compound in the plant, gives it this flavor. In small quantities sorrel makes a bright, lip-puckering addition to salads, soups, or sauces—but too much oxalic acid can be mildly toxic and hard on the digestive tract