Preheat the oven to 350°F (175°C). Spray a 9x13-inch casserole dish with non-stick cooking spray and set aside.
In a large skillet over medium-high heat, cook the ground beef and diced onion until the beef is browned and the onion is softened, about 6–8 minutes. Drain excess fat if desired.
Add the minced garlic to the skillet and sauté for 1–2 minutes until fragrant.
Stir in the taco seasoning and 1/2 cup water; cook until well combined and slightly thickened.
Add the fire-roasted diced tomatoes with their juices, green chiles, drained black beans, and drained corn to the beef mixture; heat through, about 2–3 minutes. Remove from heat.
Line the bottom of the prepared baking dish with 6 corn tortillas, overlapping slightly to cover the base.
Spread one-third of the meat mixture evenly over the tortillas, then sprinkle with one-third of the shredded cheese.
Repeat two more times with 6 tortillas, meat mixture, and cheese to make three layers, finishing with cheese on top.
Cover the dish with aluminum foil and bake for 30 minutes, until the casserole is bubbly and the cheese is fully melted.
Remove foil and let the casserole sit 10 minutes to set. Garnish with diced tomato and chopped cilantro, and serve warm with sour cream and avocado on the side.