Place onion flat-sides down and thinly slice crosswise. Combine vinegar, water, sugar, cumin, chili powder, and salt in container with lid. Shake well to mix, add onion, and shake to coat onion thoroughly. Place in refrigerator and marinate overnight, shaking onions in marinade 2 or 3 times. To assemble salad, heap lettuce on 4 plates. Arrange tomato slices on salads. Lift onions from marinade and heap into center. Halve, seed, and peel avocado. Slice each half lengthwise into thin slices, keeping slices attached at one end. Fan out and divide each fan onto two salads. Sprinkle salads with cilantro and chile slices. Add olives. Serve with Picante Vinaigrette. Makes 4 servings.Picante Vinaigrette: in jar, combine 1/4 cup olive oil, 2 Tbsp. red wine vinegar, 2 Tbsp. medium picante sauce, 1 Tbsp. sugar and 1 tsp. dried oregano; shake well. Makes 1/2 cup dressing.