Preheat the oven to 350°F (175°C) and spray a 9x13-inch baking dish with nonstick cooking spray; set aside.
In a large skillet over medium-high heat, cook the ground beef and diced yellow onion, breaking up the meat, until the beef is browned and the onion is softened, about 6–8 minutes.
Add the minced garlic and cook 1–2 minutes more until fragrant.
Stir in the taco seasoning and 1/2 cup water and simmer briefly until combined.
Add the fire-roasted diced tomatoes with their juices, green chiles, drained black beans, and drained corn to the skillet and heat through, stirring to combine.
Place 6 corn tortillas in the bottom of the prepared baking dish, overlapping slightly to cover the surface.
Spread one-third of the meat and bean mixture over the tortillas, then sprinkle with one-third of the shredded jack and cheddar cheese.
Repeat two more times with tortillas, meat mixture and cheese to create three layers, finishing with cheese on top.
Cover the dish with aluminum foil and bake for 30 minutes, until bubbly and the cheese is fully melted.
Remove from the oven and let the casserole rest for 10 minutes, then garnish with diced tomato and chopped cilantro and serve with sour cream and avocado on the side.