Heat the oven to 375°F (190°C) and lightly grease a 9 x 13-inch baking dish. Set the prepared dish aside while you cook the beef mixture.

Brown the ground beef and diced onion in a large skillet over medium-high heat, breaking the meat into small pieces as it cooks. Drain any excess fat before adding the remaining filling ingredients.

Stir in the taco seasoning, refried beans, and undrained diced tomatoes with green chiles. Cook for 2–3 minutes, stirring until the mixture is evenly combined and hot.

Spread a small amount of red enchilada sauce across the bottom of the baking dish. Add tortillas over the sauce, trimming or slightly overlapping them so the base is covered.

Add approximately one-third of the beef-and-bean mixture, one-third of the remaining enchilada sauce, and one-third of the shredded cheese. Repeat the tortilla, filling, sauce, and cheese layers twice more, finishing with enchilada sauce and cheese on top.

Scatter sliced black olives over the top if using, then cover the baking dish with foil. Bake for 20 minutes, remove the foil, and bake for another 10–15 minutes until the cheese is melted and bubbling.

Let the casserole rest for 5–10 minutes before slicing. Garnish with sliced green onions and serve with sour cream, cilantro, or avocado if desired.