Put the head of lettuce on a flat surface and cut in half at the equator. Transfer the lettuce to a small tray and put it in the freezer.
In a medium bowl, whisk together the Dijon mustard, vinegar
, and pinch of salt. Slowly whisk
in the olive oil and then add the tarragon
. Taste for seasoning and set aside.
Allow the lettuce to remain in the freezer until just before serving. They can remain in there for 4 to 6 hours, if necessary. When ready to serve, transfer to a platter and drizzle
each half with the dressing
. Serve immediately.