Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch (23x33 cm) baking dish and set it aside.
In a large frying pan over medium heat, cook the ground beef until fully browned, breaking it apart with a spoon or spatula. Drain off any rendered fat.
Sprinkle the taco seasoning mix over the cooked meat, then pour in the measured water. Stir well and let the mixture simmer for about 2–3 minutes, or until it slightly thickens.
Fold in the black beans, corn, and the drained diced tomatoes with green chilies. Stir to combine and continue to cook for an additional 2 minutes, allowing the flavors to meld.
To assemble, begin by laying 2–3 tortillas at the bottom of the prepared baking dish, trimming them if necessary to fully cover the base. Spread a portion of the meat and vegetable mixture evenly over the tortillas. Dollop with sour cream and sprinkle a mix of both cheeses over the top.
Continue layering with more tortillas, followed by the meat mixture, sour cream, and cheese, repeating until all components are used. Make sure to finish with a generous layer of cheese on the top.
Cover the dish loosely with aluminum foil and bake in the preheated oven for 20 minutes. Then, remove the foil and continue baking for another 10 minutes, or until the top is golden and bubbling.
Allow the casserole to rest for about 5 minutes before cutting. Garnish with scallions and fresh cilantro if using. Serve warm with additional sour cream or a side of salsa, as desired.