Preheat oven to 350 degrees.
Open and drain the cans of chicken, and break up with a fork, or alternately, shred any other type of chicken. Place chicken in a bowl. Add about 1/4 cup of the shredded cheese, and the chopped onion and green chilies if using. Mix it up and pour in about a few tablespoons of the enchilada sauce. You want the mixture to be wet but not soupy.
Spray your pan with nonstick cooking spray so nothing sticks to the bottom. Pour a little of the enchilada sauce on the bottom of the pan and then line it with six corn tortillas. They will overlap. Pour on a little more of the sauce and spread it over the tortillas. Spread 1/2 of the meat mixture over the tortillas and top with a little more sauce, and then a layer of cheese. Add another layer of six tortillas, sauce, meat mixture, sauce and cheese. Top with one more layer of tortillas, the rest of the sauce and the rest of the cheese.
Bake for about 30-45 minutes, covered, or until the casserole is hot and bubbly and the cheese is melted. You can remove the cover for the last 10-15 minutes if you’d like for the cheese to get a little crusty.
Remove from oven, and let cool slightly before cutting. Serve with the toppings of your choice.