In a small bowl, whisk together the vinegar, shallot, mustard, salt, and pepper. Then add the olive oil in a slow stream, whisking until emulsified. Whisk in tarragon.
In a large salad bowl, toss the frisee, Boston lettuce, arugula, and watercress and then season with salt and pepper.
NOTE: Greens can be washed, trimmed, and torn one (1) day ahead and chilled in a sealed plastic bag lined with paper towels.
DO NOT OVERDRESS YOUR SALADS - Too much salad dressing will weight down the salad ingredients and mask their flavors. The dressing's role is to highlight not to overpower the salad ingredients. A general rule is 1/4 cup of dressing for 6 cups of greens. As soon as your salad is mixed, taste a leaf to see if there is sufficient dressing. If not, drizzle some more over the salad, a tablespoon at a time; toss and taste again.
Makes 8 servings.
Mixed Green Salad with Tarragon Vinaigrette Recipe: https://whatscookingamerica.net/lindasandberg/salads/mixedgreens-tarragonvingaigrette.htm