Preheat your oven to 350°F. Prepare a 9x13-inch casserole dish with a light coating of nonstick cooking spray.
In a large skillet, cook the ground beef and onion over medium heat until the beef is no longer pink and the onions are tender, about 8-10 minutes.
Add the minced garlic to the skillet. Cook and stir until fragrant, about another 1-2 minutes.
Mix in the taco seasoning and water, stirring until the beef mixture is evenly coated with the seasoning.
Fold in the canned tomatoes with their juices, green chilies, and black beans. Cook until the mixture is warmed through, about 4-5 minutes. Remove from heat and set aside.
Create the first layer in your casserole dish by arranging 6 corn tortillas in the bottom so they overlap slightly.
Spoon 1/3 of the beef mixture over the tortillas. Sprinkle with 2 cups of shredded cheese.
Continue layering the remaining tortillas, beef mixture, and cheese. End with a final sprinkle of cheese on top.
Cover the dish with aluminum foil. Bake until the casserole is hot and bubbly and the cheese has melted, about 30-40 minutes.
Garnish the baked casserole with diced tomatoes and green onions, then let it rest for 10 minutes to set before slicing.
Serve warm with a dollop of low-fat sour cream, if desired.