Add defrosted chicken breast, enchilada sauce, diced sweet onion to crock pot. If desire, can also add one or more of these spices to enhance taste - cumin, chili powder, coriander, garlic (fresh or powdered form), salt, or pepper. Add cayenne powder for additional spiciness.
Cook on high for 4 hours. Remove chicken breasts from crockpot and use a fork shred chicken. Bring shredded chicken breast back to crockpot and continue cooking for another 30 minutes.
Pre-heat oven to 375 degrees F.
To assemble casserole, spoon a few spoonfuls of the juice from crock pot to the bottom of a 9"x13" casserole dish.
Next, layer casserole with 6 corn tortillas.
Next, layer with 1/3 of the shredded chicken (with some sauce).
Then sprinkle with 1/2 cup of corn, spread evenly.
Then add 1 cup of cheese, spread evenly
Repeat step 5-8 for another two times. There should be a total of 3 layers. Note that there should be a lot of cooked sauce/juice not used - do not spoon all of the enchilada sauce into casserole.
Bake casserole for 20-25 minutes.
Garnish with chopped cilantro. Optional (but highly recommended!): Serve with chips, rice, queso, and/or guac, and enjoy!