In a small bowl, combine the Greek yogurt, lemon juice, olive oil, dried oregano, salt, and pepper. Mix well and set aside.
Heat 1 tablespoon of olive oil in a skillet over medium heat. Add the thinly sliced chicken breasts, season with salt and pepper, and cook for about 5-6 minutes per side, or until browned and cooked through. Remove the chicken from the skillet and set aside on a paper towel-lined plate.
In the same skillet, add the thinly sliced onion and minced garlic. Cook over medium heat for 3-4 minutes, or until the onions are soft and translucent.
Warm the pita breads in a toaster or oven until they are soft and pliable.
Spread a generous amount of the prepared yogurt sauce on each warm pita bread. Top with the cooked chicken and sautéed onions.
Sprinkle the assembled chicken pitas with freshly chopped herbs. Serve immediately and enjoy!