The essence of lemon brings out the very best in fresh asparagus. Here, the lemony tartness comes from delicate sorrel greens, which are used to flavor the fresh-tasting yogurt-mayonnaise dressing. Look for sorrel especially in the spring.
For dressing, in a small bowl combine yogurt, mayonnaise, sorrel, green onion, and, if desired, lemon-pepper seasoning. Cover and chill until ready to serve.
Fold a 36x18-inch piece of heavy foil in half to make a double thickness of foil that measures 18x18 inches. Place asparagus in center of foil. Fold up edges of foil slightly; drizzle asparagus with the water. Bring up 2 opposite edges of foil; seal with a double fold. Fold remaining ends to completely enclose asparagus, leaving space for steam to build.