Preheat the oven to 375 degrees F (190 degrees C). Lightly coat a 9x13-inch baking dish with nonstick cooking spray.
Stir together chicken, beans, corn, and peppers in a bowl.
Whisk together enchilada sauce, sour cream, salt, and pepper in another bowl until well combined.
Arrange 6 tortilla halves in the prepared baking dish to mostly cover the bottom (it’s okay to leave open spaces). Spread ¼ the enchilada sauce mixture over the tortillas (about ⅔ cup).
Evenly top with ⅓ of the chicken mixture (about 2 cups), then sprinkle with ½ cup Pepper Jack cheese. Repeat this layering step 2 more times.
Arrange remaining 6 tortilla halves on top. Spread with remaining ¼ enchilada sauce mixture and sprinkle with remaining ½ cup Pepper Jack cheese.
Bake in the preheated oven until heated through and cheese is melted, 25 to 28 minutes.
Let stand for 5 minutes before serving. Garnish with cilantro and avocado.
