This is like a vaguely Mexican lasagna, with corn tortillas instead of noodles. It's just different and exotic enough to be seen as an alternative to tomato-based pasta dishes for a crowd but not scary, so it's good for something like a teen dinner.
Saute the onion in the oil. Mix it with all ingredients through the chicken. Spray a 10x13 pan (or a disposable half-size steam table pan, which is 10x12, but deeper than a regular pan) with cooking spray. Layer the chicken mixture with the cheese and tortillas, starting with the chicken mixture and using 6 tortillas for each of 3 layers. Sprinkle additional cheese on top if desired.
Bake, covered, at 350° for 30-40 minutes or until heated through. Let stand for 10 minutes before cutting if possible. Just as with lasagna, this delay will make the layers set up better and help prevent puddles on the plate. Be sure, by the way, if you're using the disposable pan, to put it on a cookie sheet before you put it in the oven, and take it out of the oven with the cookie sheet still underneath it. Those disposable pans are great--I have a ton of them that I bought at Costco, and they come in very handy--but they're not what you'd call sturdy. You don't want your lovely pan of casserole to fold in half as you take it to the table!
A Mexican-themed salad would go well with this--take a look in the "salads" section for a dressing idea and list of ingredients. You could serve cornmeal muffins, too, but since the casserole itself has corn tortillas it's probably not necessary. A nice dessert is Mexican brownies, also in the recipes.