In a large skillet, heat the 2 tablespoons of olive oil over medium heat. Once hot, add the chopped red onion and sauté for about 3-4 minutes, or until the onion becomes translucent and fragrant.
Add the pound of lean ground beef to the skillet with the onions. Cook until the beef is browned, breaking it apart with a spoon as it cooks. This should take about 5-7 minutes. Drain any excess fat if necessary.
Sprinkle the 2 tablespoons of low sodium Mexican seasoning over the beef and onion mixture. Stir well to combine and let the flavors meld for another 2-3 minutes.
Preheat your oven to 350°F (175°C). In a 9x13 inch baking dish, start layering your casserole. Place 4 corn tortillas on the bottom, followed by half of the beef mixture, half of the enchilada sauce, and a cup of the shredded cheese. Repeat the layers, finishing with the remaining tortillas, sauce, and cheese on top.
Cover the baking dish with aluminum foil and place it in the preheated oven. Bake for 20 minutes covered, then remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden brown.
Once baked, remove the casserole from the oven and let it cool for a few minutes before slicing. Serve warm, garnished with fresh cilantro, diced tomatoes, or avocado if desired.