Bring 2 cups water to a boil in a medium-size saucepan, add the chicken and 1/2 teaspoon salt, skim off the foam that rises as the water returns to a boil, partially cover and simmer over medium heat—23 minutes for the dark meat, 13 minutes for the breast. If there is time, cool the chicken in the broth.
Boil the potatoes and carrots in salted water to cover until they are just tender, 12 to 15 minutes. Rinse for a moment under cold water, strip off the potato skins, if you wish, then cut the potatoes and carrots into 3/8-inch dice. Place in a large mixing bowl.
Skin and bone the chicken, then tear the meat into large shreds and add to the potatoes. Skim off all the fat on top of the broth, then measure 3 tablespoons of broth into a small bowl. Stir in the vinegar, oregano and the remaining 1/2 teaspoons salt. Pour the dressing over the chicken mixture and add the sliced chiles chipotles and chopped onion. Stir, cover and let stand for 45 minutes, refrigerated or at room temperature.
Shortly before serving, mix the sliced lettuce and diced avocado into the chicken mixture. Drizzle with oil and toss lightly. Taste for salt. Line a serving platter with the remaining romaine leaves and pile on the chicken mixture. Decorate with the onion rings and serve.
Variations on the Salpicon Theme: The proportions and selection of the main ingredients should be kept loose. Meats such as ham and leftover pork or beef roast can easily be substituted for the cooked chicken. Or replace the cooked chicken with 4 or 5 ounces (about 2/3 cup) diced smoked chicken.