Preheat oven to 350 degrees. Lightly grease a 9x13 casserole dish and set aside.
In a skillet over medium heat, brown ground beef and onions. Remove from heat. Add black beans and taco seasoning, stirring to combine.
In prepared 9x13 dish, layer ingredients. Add ¼ of enchilada sauce to bottom of dish, then add a layer of 3 tortillas. Next, add half of the meat/bean mixture, spreading evenly in a thin layer. Then, sprinkle with ⅓ of your shredded cheese. Repeat this sequence one more time: sauce, tortillas, meat, and cheese.
Next, add another ¼ of your enchilada sauce, a layer of tortillas, the remaining shredded cheese, and the remaining enchilada sauce.
Lightly cover with foil and bake for 25 minutes. Remove foil and bake for 5 more minutes.
Remove pan from oven and allow to cool for 10 minutes before serving.
Optionally, top your casserole with: sour cream, plain greek yogurt, pico de gallo, salsa, fresh tomatoes, cilantro, parsley, green onion, guacamole, fresh sliced avocado, or jalapeños.
Leftovers will keep in the refrigerator for 2-3 days and reheats well.
Casserole can be made ahead of time. Assemble, then cover and refrigerate or freeze. Remove from freezer the night before to thaw in the refrigerator. Bake as directed and serve.