Heat the oven to 350°F and lightly grease a 9-by-13-inch baking dish.

Cook the ground beef and diced onion in a large skillet over medium-high heat for 7–10 minutes, breaking up the meat as it cooks, until the beef is browned and the onion is tender. Drain any excess fat.

Stir the taco seasoning and water into the cooked beef. Simmer for 2–3 minutes, stirring occasionally, until most of the liquid has been absorbed.

Spread a small amount of enchilada sauce over the bottom of the prepared baking dish. This first layer helps keep the tortillas moist.

Arrange a layer of quartered corn tortillas over the sauce. Add about one-third of the beef, one-third of the refried beans, some drained tomatoes or optional green chiles, enchilada sauce, and cheese.

Repeat the tortilla, beef, bean, tomato or chile, sauce, and cheese layers two more times. Finish with enchilada sauce and the remaining shredded cheese on top.

Cover the baking dish with foil and bake for 20 minutes. Remove the foil and continue baking for 10–15 minutes, until the casserole is hot and bubbling and the cheese is fully melted.

Let the casserole rest for 5–10 minutes before serving. This gives the sauce time to thicken so the layers hold together more cleanly.