In a mixing bowl, combine the chopped spinach, shredded cheese, chopped onion, and black beans.
Spread a thin layer of enchilada sauce on the bottom of a baking dish.
Place a layer of corn tortillas over the sauce.
Spread a portion of the spinach mixture over the tortillas, then top with some enchilada sauce. Repeat layers until all ingredients are used, finishing with a layer of cheese on top.
Bake in the preheated oven for 30 minutes, or until the cheese is bubbly and golden brown.
Let it cool for a few minutes before serving.
Notes:
You can add some jalapeños for extra spice if you like.