Preheat the oven to 200 degrees Celsius then grease a baking dish with cooking spray and set aside. Place a non-stick skillet pan over medium heat then add in the oil. Once the oil is hot, add in the onions and some sprinkles of salt and pepper to taste then cook for about five minutes until it becomes soft.
Next, add in the garlic, black pepper, chili powder, and cumin, stir properly to combine then cook for about thirty seconds until it becomes fragrant. Add in the tomato sauce and water, stir to combine then bring the sauce to a simmer. Cook the sauce for about five minutes with an occasional stir until it thickens.
Using a large mixing bowl, add in the shredded chicken, the enchilada sauce, chili peppers, cilantro and 1 cup of cheese then mix properly to combine. Place the mixture into the greased baking sheet, top with the rest of the cheese then place the baking sheet into the preheated oven.
Bake the casserole for about ten minutes until the cheese melts. Serve.