Step 1 In a medium bowl, stir together the vinegar, salt and pepper until the salt dissolves, then stir in the mustard. Pouring the oil in a slow stream, whisk it into the mustard-vinegar mixture until well incorporated, then stir in the tarragon. This makes about two-thirds cup vinaigrette, more than is needed for the remainder of the recipe; the dressing will keep, covered and refrigerated, up to 5 days.
Step 2 Combine the lettuce leaves in a large bowl, adding the dressing, a little at a time, until the leaves are very lightly coated. Divide the lettuces among 6 plates. Serve immediately.