Heat a grill, broiler, or grill pan to medium. Season the chicken all over with the chili powder, cumin, oregano, and ½ teaspoon of the salt. Coat the cutlets with cooking spray, and then grill, broil, or sear them until browned on both sides and the juices run clear (165ºF internal temperature), 3 to 4 minutes per side. Remove the chicken to a cutting board, let it cool slightly, and slice each cutlet into three or four pieces.
Place the flatbreads on the grill, under the broiler, or in a single layer in the pan and cook on medium until heated through, 1 to 2 minutes per side.
In a small bowl, combine the avocado, mayonnaise, lime juice, cilantro, and remaining ¼ teaspoon of salt. Spread the mixture all over the bubbly side of the warm flatbreads. On one half of each spread bread, arrange the chicken, cheese (if using), scallions, tomato, and lettuce. Fold the other half of each flatbread over to cover the toppings, and serve.
Tip: Flatbread makes a sandwich way more fun. Even the kids will love this one, which combines the flavors of a chicken taco with a southwest panini. Having trouble finding naan style flatbread? Flatbread is available by special request at your local Panera Bread bakery-cafe.