Preheat the oven to 350°F. Lightly grease a medium baking dish. Season the chicken breasts on both sides with garlic salt and arrange in the dish. Bake for 45 minutes until cooked through (internal temp 165°F). Cool, then shred with two forks. Set aside.
Char each torn tortilla half over a gas burner flame using metal tongs for about 1 minute until lightly puffed with char marks. Alternatively, toast in a dry cast iron skillet over high heat for 30–45 seconds per side. Set aside.
Pour about ½ inch of enchilada sauce into the bottom of your baking dish and spread to coat fully. Arrange 6 charred tortilla halves over the sauce in a single layer.
Top with half the shredded chicken, half the sour cream, one-third of the cheese, and one-third of the remaining enchilada sauce. Spread each layer evenly.
Repeat: add another layer of 6 tortilla halves, remaining chicken, remaining sour cream, another third of the cheese, and another third of the sauce.
Arrange the final 6 tortilla halves on top. Coat them thoroughly with the remaining enchilada sauce, covering all edges. Sprinkle the last of the Monterey Jack generously over the top.
Cover tightly with foil and bake for 45 minutes at 350°F, until the cheese is fully melted and the sauce is bubbling at the edges. Rest for 8–10 minutes before slicing and serving.