Preheat your oven to 375°F (190°C) and lightly spray a 9x13-inch baking dish with non-stick cooking spray. Pour about ¼ cup (60ml) of enchilada sauce into the bottom of the dish and spread into a thin, even layer to prevent the tortillas from sticking.
Heat a large skillet over medium-high heat, add the ground beef, and cook for 5 to 6 minutes, breaking it into crumbles, until no pink remains. Carefully drain the excess grease from the skillet.
Reduce heat to medium, add the diced onion to the beef, and cook for 3 to 4 minutes until soft and translucent. Add the minced garlic and cook for 1 minute more until fragrant.
Sprinkle the taco seasoning over the beef mixture, pour in the beef broth, and stir well. Simmer on medium-low for 2 to 3 minutes until most of the liquid is absorbed, then stir in the black beans and corn and heat through for 1 minute. Remove from heat.
Stack the tortillas in a damp paper towel and microwave for 30 to 45 seconds until pliable, or warm them individually in a dry skillet for 20 to 30 seconds per side. Keep covered with a kitchen towel to stay soft.
Place 4 corn tortillas in an even layer over the sauce in the baking dish, overlapping slightly as needed. Spread half of the beef filling over the tortillas, pour one-third of the remaining enchilada sauce over the beef, and sprinkle with ¾ cup of Mexican blend cheese.
Place another 4 tortillas over the cheese, spread the remaining beef filling evenly on top, pour another one-third of the enchilada sauce over the beef, and sprinkle with another ¾ cup of Mexican blend cheese.
Lay the last 4 tortillas over the cheese, pour the remaining enchilada sauce over the top and spread it edge to edge. Sprinkle the remaining Mexican blend cheese and all of the reserved sharp cheddar evenly over the top.
Cover the baking dish tightly with aluminum foil and bake at 375°F (190°C) for 25 minutes. Remove the foil and bake uncovered for an additional 10 minutes until the cheese is melted, bubbly, and beginning to turn golden.
Let the casserole rest for 5 to 10 minutes before slicing to allow the layers to set. Garnish with cilantro, green onions, jalapeños, and sour cream, and serve hot directly from the baking dish.