Preheat oven to 350°F (175°C). Grease a 9×13-inch baking dish with cooking spray.

Cook the meat filling:
In a large skillet over medium heat, brown the ground beef and onion until the meat is no longer pink (about 6–8 minutes).
Stir in garlic and cook for 1 minute. Add taco seasoning and water; stir to combine.
Add diced tomatoes (with juices), green chiles, and black beans. Stir and let heat through for 3–4 minutes.

Assemble the layers:
Layer 6 corn tortillas on the bottom of the baking dish, overlapping slightly.
Spread one-third of the meat mixture on top, then sprinkle with 2 cups of cheese.
Repeat the layering (tortillas, meat, cheese) two more times for 3 total layers.

Bake:
Cover tightly with foil and bake for 30–40 minutes, until bubbly and cheese is melted.

Rest and serve:
Let sit for 10 minutes before slicing. Top with diced tomatoes, green onions, and sour cream.