Watercress with Pistachios

In a large bowl, whisk together 1 teaspoon Dijon mustard, 1 teaspoon lemon juice, 2 tablespoons olive oil, and 1/8 teaspoon kosher salt. Add 1 bunch watercress, 1/4 cup shelled pistachios, and the zest of 1 lemon. Toss well.