Preheat to 350°F. Lightly grease a 9x13-inch baking dish.
In a large bowl, stir together the sour cream, chopped onion, green chiles, garlic salt, and chicken. Mix until well combined.
Spread a thin layer of the sour cream chicken mixture on the bottom of the prepared dish.
Add a layer of torn corn tortillas to cover the chicken mixture.
Drizzle with a small amount of red enchilada sauce (about 2 tablespoons).
Add another layer of the sour cream chicken mixture, then sprinkle with half of the shredded cheese.
Repeat the layering process, ending with ½ the shredded cheese (2 cups) on top.
Cover loosely with foil and bake for 25 minutes. Remove foil and bake an additional 5 minutes, or until bubbly and the cheese is fully melted.
Let rest for 5-10 minutes before serving. Garnish with fresh cilantro, diced avocado, or a dollop of sour cream if desired.