I'm splitting the ingredients list into two parts -- the first are the mandatory ingredients (though I suppose you could omit the onion). The second are recommended additions -- feel free to adjust, change, add as you see fit. When assembling the dish, it helps to think of it as "Mexican lasagna," since the tortillas are layered just like lasagna noodles
2cupsred enchilada saucesee below, or one large can low-sodium enchilada sauce
1 1/2cups1 can black beans
1can no-fat low-sodium refried beans
10-12corn or 100% whole wheat tortillasread ingredients carefully; look for tortillas with short ingredient lists!
1smallcan of diced green chiles
3 to 5clovescrushed garlic
1/2 to 2largediced jalapeñoscheck for spiciness, jalapeños vary considerably
bunchfresh spinachfrozen or canned works, too
1cuppotatoessliced into 1/4" rounds and blanched
1cupshredded cheesepart-skim mozarella, jack, or "Mexican blend," all work well
sliced avocadofor garnish/topping
Heat the oil in a large saucepan over medium heat. Saute the diced onion for 3-4 minutes, until it starts to soften.
Reduce heat to medium-low, and add the enchilada sauce, black beans, and refried beans to the saucepan. Stir slowly until mixed thoroughly.
Add any "extra goodies" to the sauce and continue heating and stirring until it starts to simmer. Be sure to stir continuously, else it may start to splatter as it comes to a boil.
The goal is not necessarily to cook the veggies and sauce, rather to be sure it's evenly mixed and hot. If using fresh spinach, you'll want the leaves to be wilted/softened, so they don't take up too much space.
Once everything is mixed well, turn off heat and start layering the casserole in a lightly greased 9x13 baking pan.
First, add thin layer of the sauce to the bottom (if you start with tortillas first, they may burn). Add a layer of tortillas (see diagram, below). Then alternate sauce and tortillas until there's about 1/4" of space left (or you run out of sauce or tortillas!), ending with a layer of sauce on top. Be sure all the tortillas are covered by sauce; any tortilla that's not covered will dry out.
Sprinkle the cheese on top, and cover with aluminum foil. "Tent" the foil so it's not resting on the cheese (this makes it easier to remove later).
Bake at 350°F for 40 minutes. (If doubling the recipe, or using a deeper pan, you may want to bake a little longer). Remove foil, and bake for another 10 minutes, until cheese begins to brown. Allow to cool for at least ten minutes before serving.
Tear tortillas in halves or quarters as necessary to create a single layer. I usually start with one whole tortilla in the middle, and then tear another in half to create flat edges, which go on the outside of the pan. I may tear a third one in quarters, and use that to fill in the gaps. It's good if they overlap a little.
Using canned enchilada sauce speeds things up, but the sodium content is usually frustratingly high. You can also use canned tomato puree or chunks (or fresh diced tomatoes, simmered for a few minutes), with cumin, chile powder, oregano, garlic/garlic powder and any other spices as you see fit. It should be thin; the tortillas will absorb that extra moisture.