Toss chicken with 2 teaspoons of the ranch dressing mix.
Heat the oil in a large skillet over medium heat; sauté chicken until chicken cooked through.
Meanwhile, in a bowl, combine chopped lettuce, bell pepper and onion; set aside.
In a small bowl or cup, combine sour cream, mayonnaise, milk and remaining salad dressing mix; pour over lettuce mixture and toss well to coat; spoon into pita halves.
Place pitas on plastic wrap or aluminum foil and seal; refrigerate overnight or until ready to serve for lunch.