Place the scallions and 2 quarts water in a large, deep skillet and bring to a boil. Add a pinch of salt and the chicken breasts, smooth side down. Reduce the heat to a bare simmer. Place a plate or lid (smaller than the pan) directly on top of the chicken to keep it submerged and cook gently for 20 minutes, until chicken is firm. Let cool and chill, if desired.
Meanwhile in a bowl, mix the yogurt, lemon juice, cucumber, cilantro, garlic, cumin, and turmeric. Season to taste with salt and pepper.
Slice the chicken and arrange on plates. Spoon the sauce on top. You can wrap the chicken in lettuce leaves, if desired,