Here's what you need:
Okay, maybe not that simple. You do need to open those cans of mushroom soup and add the chopped onion and green chiles to make a sauce:
Green chiles are easy to find in cans. Ortega and Old El Paso both sell 4-oz cans of chopped green chiles that are just about the right amount for this recipe. The canned green chiles come in both hot and mild versions, so that you can tailor the spiciness to the group that is going to eat the enchiladas. For this particular casserole, I used really hot frozen green chiles that are not universally available.
I generally heat up the sauce a little to make it smoother and more spreadable. Once the sauce is ready and the cheese is grated, putting the casserole together takes minutes:
Start assembling the casserole by putting a thin layer of sauce on the bottom of the pan. I usually spray the baking pan with Pam cooking spray before adding the sauce. This particular casserole is in a foil pan because I was taking it to someone else's house for a communal meal, but I usually make it in a glass 9x13 pan.
Then put another layer of sauce on top of the tortillas.
My mother used to make this casserole all the time, not only for family meals but also for potlucks and other communal meals. Not being vegetarian (and not keeping kosher), she would often add shredded chicken to her sauce. I've never tried making it with any of the chicken analog products that are on the market, but it would probably still be tasty.
Green Chile Enchilada Casserole
2 cans cream of mushroom soup
1 can evaporated skim milk*
1 large onion, chopped
1 small can chopped green chiles
12-16 ounces monterey jack cheese, grated
6-8 ounces mild cheddar cheese, grated
12-16 corn tortillas, torn into quarters
Combine the soup, milk*, chopped onions, and green chiles. Stir until fairly smooth - some small lumps are no problem.
*Note: I almost never add the milk to the sauce because I find that it makes the casserole too soupy. Sometimes if it looks like I won't have enough sauce for the final layer, I will stretch the sauce a bit by adding small amounts of canned milk. At least one of my sisters always adds the canned milk because she likes the soupier casserole. So experiment -- find what works for you.
In a 9x13 pan, spread a thin layer of the soup mixture over the bottom to coat. Use about 1/3 of the tortillas to cover the bottom, add a layer of the soup mixture, then a layer of cheese.
Add two more layers of tortillas, then soup, then cheese.
Bake at 350 to 375 degrees for 45 minutes to 1 hour, until casserole is bubbly and cheese is nicely browned.
These cooking instructions are almost infinitely variable. You can bake it longer at a lower temperature, or for a shorter time at a higher temperature. You can cook it for 30 minutes or so, then continue cooking it later.