Preheat the oven to 375°F. Lightly grease a 9x13-inch baking dish.
Add the ground beef to a large skillet over medium-high heat. Cook, breaking the beef apart as it cooks, until browned and cooked through. Drain any excess grease.
Stir the taco seasoning and diced green chiles directly into the cooked ground beef. Cook for another 1–2 minutes. No additional water is needed.
Spread a thin layer of enchilada sauce over the bottom of the prepared baking dish.
Arrange flour tortillas over the enchilada sauce, overlapping or cutting them as necessary to cover the bottom.
Spread half of the seasoned ground beef over the tortillas. Top with half of the black beans, red onion, enchilada sauce, and shredded cheddar cheese.
Add another layer of flour tortillas. Repeat with the remaining ground beef, black beans, red onion, enchilada sauce, and cheddar cheese.
Finish with a final layer of tortillas. Pour the remaining enchilada sauce over the tortillas and top with the remaining cheddar cheese.
Cover loosely with aluminum foil and bake for 20 minutes.
Remove the foil and bake for another 5–10 minutes, or until the casserole is heated through and the cheese is melted and bubbly.
Remove from the oven and let the casserole rest for 10 minutes before slicing.
Serve warm with sour cream, avocado, cilantro, tomatoes, jalapeños, or your favorite taco toppings.