To make the dressing, combine the rice vinegar, vegetable oil, soy sauce, sesame oil, grated ginger, sugar and 1/2 teaspoon salt in a small bowl and mix well. Set aside.
Combine the remaining salt and the five-spice powder and sprinkle the mixture over the duck. Place a nonstick frying pan over high heat until hot. Place the duck, skin side down, in the pan and cook until golden brown, about 3 minutes. Turn the breasts, reduce the heat and cook until medium-rare, 6 to 7 minutes. Remove the duck from the pan, place on a cutting board, and let rest for 5 minutes. Do not cover the duck or the skin will not be crisp.
In a large bowl, combine the watercress, snow peas and cabbage. Pour the dressing over the top and toss to coat. Divide the salad among 4 serving plates. Cut the duck into slices on the diagonal. Arrange the slices on each salad and serve.