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.