2 1/2 tablespoon lemon juice plus the zest of one lemon (zest before juicing)
2 tablespoons lime juice plus the zest of 1 lime (zest before juicing)
1 tablespoon honey
1 teaspoon Dijon mustard
Sea salt (I recommend Maldon) or kosher salt and freshly ground black pepper
6 tablespoons canola oil
For the Salad
3 heads butter lettuce (also called Bibb, Boston or living lettuce)
1/2 cup fresh tarragon leaves, roughly chopped
Make the Dressing: Whisk lemon juice and zest, lime juice and zest, honey, and dijon in a small bowl. Slowly drizzle in the oil, whisking constantly, to emulsify the dressing. Season, to taste, with salt and pepper. Set aside.
Make the Salad: Cut the root end off the lettuces and gently wash and dry the leaves. I like to rinse leaves separately, under the tap, and spin them dry in my salad spinner. Tear the larger leaves into smaller pieces. Pile leaves into a large bowl. Sprinkle in the tarragon and toss it gently with the lettuce. Just before serving, toss the salad with just enough dressing to coat. Serve immediately.