On a large plate drizzle the chicken with 1 tablespoon of the oil, sprinkle it with the scallions, 1 teaspoon of the thyme, and salt and pepper to taste, turning it to coat it well, and let it marinate at room temperature, turning it once, for 30 minutes. In a small bowl whisk together the vinegar, the shallot, the remaining 1 teaspoon thyme, and salt and pepper to taste, add the remaining 4 tablespoons oil in a stream, whisking, and whisk the vinaigrette until it is emulsified. Grill the chicken on an oiled rack set 5 to 6 inches over glowing coals for 4 to 5 minutes on each side, or until it is cooked through. In a large bowl toss the romaine, the watercress, the cabbage, and the mango with just enough of the vinaigrette to coat the salad well, transfer the salad to a platter, and top it with the chicken. Serve the remaining vinaigrette separately. Serves 4. Gourmet July 1993