Finely shred peel from lemon; juice lemon. In saucepan combine peel, juice, chicken, broth, garlic, and oregano; bring to simmer over medium-high heat. Reduce heat and simmer, covered, 10 minutes or until no pink remains in chicken. Drain, reserving 1/3 cup cooking liquid.
Meanwhile, chop half the cucumber; slice remaining. For dressing, place reserved cooking liquid in bowl; whisk in yogurt. Remove half the dressing and set aside. Add chicken to bowl along with chopped cucumber; toss to coat.
Layer tomato and sliced cucumber on plates. Top with chicken mixture. Drizzle with some of the reserved dressing. Season with salt and pepper. If desired, top with fresh oregano. Pass remaining dressing.