Cook the zucchini in boiling salted water until cooked but still firm and crisp, about 5 to 6 minutes. Drain, chop coarsely, and cool to room temperature. Combine the zucchini, scallions, chiles, and avocados in a large bowl. Whisk together the oil, vinegar, mustard, salt, and pepper and pour over the vegetables. Toss gently to combine and serve on lettuce leaves. Serves 4 to 6.