Preheat the oven. Preheat your oven to 350°F. Have a 9x13-inch baking dish ready.

Brown the beef and onion. Add the ground beef and chopped onion to a large 5 to 6-quart saucepan or deep skillet over medium-high heat. Cook for about 5 to 6 minutes, breaking the beef apart as it cooks, until the meat is completely browned and the onion has softened. Drain off any excess grease.

Season the beef. Pour in the water and add the taco seasoning. Stir well and continue cooking for about 3 to 4 minutes, until the seasoning is evenly distributed and the liquid has reduced slightly.

Finish the meat filling. Stir in the drained diced tomatoes and green chilies, black beans, and half of the salsa. Cook just until the mixture begins to gently simmer, then remove it from the heat.

Prepare the baking dish. Spread the remaining salsa evenly over the bottom of the 9x13-inch baking dish.

Make the first layer. Arrange 6 corn tortillas over the salsa, overlapping them slightly as needed to cover the bottom. Spread one-third of the seasoned beef mixture over the tortillas, followed by 2 cups of shredded Colby Jack cheese.

Add the second layer. Top with another 6 corn tortillas, another one-third of the beef mixture, and another 2 cups of cheese.

Finish the casserole. Add the final 6 tortillas, followed by the remaining beef mixture and the final 2 cups of shredded cheese.

Bake. Loosely cover the baking dish with aluminum foil and bake for 35 minutes. If you want the cheese slightly browned, remove the foil for the final 5 minutes of baking.

Rest before slicing. Remove the casserole from the oven and let it rest for 10 to 15 minutes. This gives the tortillas time to absorb the sauce and helps the layers hold together when sliced.

Garnish and serve. Cut into 12 portions and top with diced Roma tomatoes and sliced green onions. Add a dollop of sour cream if desired and serve warm.