One of the key ingredients to the delicate texture of this salad, is the use of Butter lettuce. This lettuce is usually sold 'living' - with its root ball, which you should keep moist in its container. Be sure not to substitute the lettuce for another variety - it's the key to the tender flavor of this salad.
If you can't find tarragon-flavored vinegar, replace with GF Champagne vinegar (but using the flavored vinegar really does make a huge difference - the flavor of this salad is really intensified!).
*Be aware that some brands of tarragon-flavored vinegar contain gluten, so be sure to check the label.