Heat oven to 375 degrees. Arrange potatoes in a roasting pan and add 1 tablespoon olive oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper; roll
potatoes to coat. Roast until potatoes are tender, about 40 minutes. Remove from oven and let stand just until cool enough to handle.
Meanwhile, whisk together lemon juice, vinegar, remaining 2 tablespoons olive oil, and remaining 1/2 teaspoon salt and 1/4 teaspoon pepper in a medium bowl; set aside.
Break each warm potato in half and place in a serving bowl. Add the vinaigrette and toss to coat. Add watercress and lemon zest, toss, and serve immediately.