1/4 cup rice vinegar (not seasoned)
1 tablespoon grated sweet onion such as Vidalia or Walla Walla (use large holes of a box grater)
1/4 cup finely grated peeled Gala apple (use small holes of box grater)
4 teaspoons soy sauce
1 teaspoon sugar
3 tablespoons vegetable oil
6 cups watercress (thin stems and leaves only; from 1 large bunch)
1 firm-ripe avocado
Stir together vinegar, onion, apple, soy sauce, and sugar until sugar has dissolved, then stir in oil.
Just before serving, toss watercress with enough dressing to coat. Quarter, pit, and peel avocado, then cut crosswise into 1/4-inch-thick slices. Gently toss with watercress.