For the salad:
1. In a resealable bag or shallow container, marinate the chicken breast in 1 cup of the dressing. Refrigerate for 15 minutes or overnight.
2. In a large mixing bowl, combine the tomatoes, cucumbers, scallions and the remaining dressing. Cover and refrigerate for 15 minutes or overnight.
3. Preheat the grill to medium.
4. Grill the chicken for about 6 minutes on each side, or until it is cooked through. Transfer the chicken to a cutting board and when the chicken is cool enough to handle, slice it into thin strips. Toss the chicken with the vegetable mixture and refrigerate.
5. Bring 1 1/2 cups of water to a boil. Stir in the couscous and a pinch of salt, cover, remove from heat and let stand for 5 minutes. Fluff the couscous with a fork and transfer it to a large serving bowl. Stir in the vegetables and chicken and season with salt and pepper. (The salad can be made in advance and stored in the refrigerator for up to 2 days. It can be served chilled or at room temperature.)
6. Garnish the salad with olives, feta cheese and mint sprigs.