Preheat the oven to 350 degrees F. Spary a 9-inch x 13-inch baking dish with non-stick cooking spray, set aside.
Place the shredded chicken, onions, taco seasoning, and water into a large skillet. Cook over medium-high heat for 5 minutes until the liquid has mostly reduced.
Add ½ cup of the enchilada sauce to the bottom of the prepared baking dish. Then place two tortillas over the sauce, covering the bottom of the dish.
Spread ⅓ of the chicken over the tortillas, then pour ½ cup of enchilada sauce over the chicken, sprinkle ¾ cup of shredded cheese over the suace, and finally top with 2 more tortillas. Repeat these steps twice more
Pour the remaining enchilada sauce over the final two tortillas and spread the sauce out to coat them. Sprinkle the remaining cheese over the casserole.
Cover the baking dish with aluminum foil and bake for 30 minutes.
Remove from the oven and carefully uncover the dish. Sprinkle with chopped cilantro and chopped tomatoes for garnish. Cut into 12 portions and serve warm with a dollop of sour cream on top.