Preheat your oven to 375°F (190°C). In a large skillet over medium-high heat, brown the ground beef along with the diced onions. Break the meat apart with a wooden spoon as it cooks. Once the beef is no longer pink, drain any excess grease from the pan. Stir in the minced garlic and taco seasoning, cooking for another minute until fragrant. Mix in the black beans and corn, then remove the skillet from the heat.

Spread about 1/2 cup of the enchilada sauce on the bottom of a 9x13 inch baking dish to prevent sticking. Place a layer of halved corn tortillas over the sauce, overlapping them slightly to cover the surface. Spread half of the beef mixture over the tortillas, pour 1/3 of the remaining sauce over the meat, and sprinkle with 1 cup of cheese. Repeat the layers: tortillas, the rest of the beef mixture, sauce, and another cup of cheese. Finish with a final layer of tortillas, the remaining sauce, and the last cup of cheese on top.

Cover the baking dish with aluminum foil and bake for 20 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and slightly golden on the edges. Let the casserole rest for 5 to 10 minutes before slicing; this helps the layers set so they do not fall apart. Top with fresh cilantro, sour cream, and avocado before serving.