Preheat the oven to 350°F (175°C) and lightly grease a medium-sized baking dish.
Season the chicken breasts with garlic salt and place them in the prepared baking dish. Bake for about 45 minutes or until the chicken is fully cooked (internal temperature should be 165°F/74°C and juices should run clear).
Once baked, remove the chicken from the oven and allow it to cool before shredding it into bite-sized pieces. Set it aside.
While the chicken cools, char the tortilla halves over an open flame using metal tongs, cooking them for about 1 minute until lightly puffed.
Pour 1/2 inch of the enchilada sauce into the bottom of the baking dish. Arrange 6 of the tortilla halves in a single layer on top of the sauce.
Layer the dish: top the tortillas with half of the shredded chicken, 1/3 of the cheese, 1/2 of the sour cream, and 1/3 of the remaining enchilada sauce.
Repeat the layers, using the remaining tortillas, chicken, cheese, sour cream, and enchilada sauce.
Once all the layers are assembled, cover the casserole and bake in the preheated oven for 45 minutes.
After baking, allow the casserole to cool slightly before serving.