This is a riff on a very famous sauce from the Troisgros restaurant in Roanne, France. It was one of the dishes that launched Nouvelle Cuisine. This version is simplified, and lightened a little from the original. Use it on fish (salmon was the original fish used), shellfish (scallops, lobster, shrimp), or on poached or slow roasted chicken breasts. Sorrel has a refreshing lemony tart/sour quality that is great with richer things like salmon and cream.