Heat the oven to 375°F and lightly grease a 9-by-13-inch baking dish.

Brown the ground beef and diced onion in a large skillet over medium heat for about 6 to 8 minutes, breaking the beef into small crumbles as it cooks. Drain excess fat thoroughly before continuing.

Add the minced garlic to the skillet and cook for 30 seconds, stirring constantly so it becomes fragrant without browning too much.

Stir in the taco seasoning, chunky salsa, drained black beans, and corn. Simmer the mixture for 2 to 3 minutes so the flavors combine and the filling becomes hot.

Spread a thin layer of the beef mixture across the bottom of the prepared baking dish. This first layer helps keep the tortillas from sticking and distributes the filling from edge to edge.

Add a layer of flour tortillas, tearing them as needed to cover the dish without leaving large gaps. Top the tortillas with part of the beef mixture and about 1/2 cup of shredded cheese.

Repeat the tortilla, beef mixture, and cheese layers until the ingredients are used, finishing with tortillas, the remaining beef mixture, and the remaining cheese. Spread the final cheese layer evenly so it melts into a golden topping.

Cover the baking dish with foil and bake for 15 minutes. Remove the foil and continue baking for another 10 to 15 minutes, until the casserole is hot and the cheese is melted and lightly browned.

Let the casserole rest for 5 to 10 minutes before slicing. Serve each portion with sour cream, extra salsa, sliced green onions, or chopped cilantro.