Heat the oven to 350°F (175°C) and grease an 11x9-inch baking dish.

Place a large skillet over medium-high heat and brown the ground beef until mostly cooked, about 5 minutes; break it up as it cooks.

Add the diced onion to the skillet and continue cooking for about 3 minutes, until the onion softens and the beef is fully browned.

Stir in the taco seasoning and 1/4 cup water, combine well, and simmer until most of the liquid evaporates, roughly 5 minutes.

Spread 1/2 cup of enchilada sauce across the bottom of the prepared casserole dish, then lay two tortillas over the sauce to form the first layer.

Spoon one-third of the seasoned beef over the tortillas, drizzle with another 1/2 cup of enchilada sauce, and sprinkle with 1 cup of shredded cheddar.

Add two more tortillas to create the next layer, then repeat with another third of the beef, 1/2 cup sauce, and 1 cup cheese.

Top with the final two tortillas, spread the remaining beef over them, pour the leftover enchilada sauce across the top, and finish by sprinkling on the remaining cheese.

Cover the dish with aluminum foil and bake for 30 minutes. Remove the foil and bake an additional 10 minutes until the cheese is melted and bubbling. Let rest a few minutes before serving.