Cut 1 peach into 1/3-inch pieces; place in large bowl. Add chicken and onion. Toss with enough dressing to coat. Cover and chill 15 minutes to 2 hours.
Add lettuce and walnuts to chicken mixture and toss to coat, adding more dressing, if desired. Mound salad on large plate. Cut remaining peach into thin wedges. Garnish salad with peach wedges and serve.