Place the head of lettuce on a flat surface and cut in half, then cut each half into 2 wedges. Transfer the lettuce to a small tray and put it in the freezer until just before serving. (It can remain in there for up to 2 hours, if necessary.)
Whisk the mustard, vinegar and a pinch of salt in a medium bowl. Slowly whisk in the olive oil and add the tarragon. Taste for seasoning.
When ready to serve, transfer the lettuce to a platter and drizzle with the dressing. Serve immediately.