Grate the cucumber and let sit in a sieve overnight in the fridge.Combine the cucumber, yogurt, garlic, oil, lemon juice, salt, and pepper in a bowl. Refrigerate for at least 2 hours.
In a bowl mix lemon juice, oregano, pepper, salt, and garlic. Pour lemon mixture over chicken breast. Roast the chicken breast in a hot pan, 4 minutes each side, until cooked.
Remove the chicken from the pan, cut it into bite-size pieces. Sweat the onions in the same pan until cooked. Mix with the chicken bites.
To serve, fill each pocket flatbread with tomato slices, chicken, iceberg salad, and top with tzatziki and a squeeze of lime juice.