Preheat the oven to 375°F. Lightly coat a 9 x 13-inch baking dish with nonstick cooking spray.
Add the ground beef and diced onion to a large skillet set over medium heat. Cook for 6 to 8 minutes, breaking the beef into crumbles, until the meat is browned and the onion has softened. Drain any excess grease.
Add the minced garlic and cook for 30 seconds, stirring constantly.
Stir in the taco seasoning, diced green chiles, and ½ cup of enchilada sauce. Cook for 1 to 2 minutes, until everything is combined and heated through. Remove the skillet from the heat.
Spread ½ cup of enchilada sauce evenly over the bottom of the prepared baking dish.
Arrange one-third of the tortilla pieces over the sauce, slightly overlapping them and filling in any gaps.
Spread half of the beef mixture over the tortillas. Spoon ½ cup of enchilada sauce over the beef and sprinkle with ½ cup of shredded cheese.
Add another third of the tortilla pieces, followed by the remaining beef mixture, ½ cup of enchilada sauce, and ½ cup of cheese.
Finish with the remaining tortilla pieces. Pour the remaining ½ cup of enchilada sauce evenly over the tortillas, making sure all exposed pieces are coated. Sprinkle the remaining ½ cup of cheese over the top.
Lightly spray one side of a sheet of aluminum foil with nonstick cooking spray. Cover the baking dish with the sprayed side facing the cheese.
Bake, covered, for 20 minutes. Carefully remove the foil and bake for an additional 15 minutes, or until the sauce is bubbling around the edges and the cheese is completely melted.
Remove the casserole from the oven and let it rest for 5 to 10 minutes before slicing. Serve with your favorite toppings.