Note: For hot peppers of any kind, be careful when handling. I have experienced a burning sensation in the past on my fingers because I used them the remove the seeds from semi-hot wax peppers. I decided to use a plastic baggie this time since I couldn't remember how hot these peppers were.
While the chicken and tomatillos are cooking, prepare the cilantro and onions. I used my mini chopper for the onions. They were very strong and made me cry more than once. You will likely need two bowls to hold the ingredients. Once the chicken is cooked, drain and chop. Add to the onion and cilantro mixture.
Repeat the layering until the casserole is full. This time I made 3 layers of filling. Add one last layer of tortillas to the top and sprinkle with cheese. I was able to make three 9x13 casseroles with this quantity of ingredients.