In a large skillet, heat the vegetable oil over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes. Then, add the ground beef, breaking it apart with a spatula. Cook until browned, about 6-8 minutes. Drain excess fat if necessary.
Once the beef is cooked, sprinkle the taco seasoning mix over the meat. Pour in the ½ cup of water and stir until the seasoning is well combined. Let it simmer for a couple of minutes until the mixture thickens slightly.
Preheat your oven to 350°F (175°C). In a large casserole dish, start layering the ingredients. Begin with a layer of corn tortillas, then spread a portion of the refried beans over the tortillas. Next, add half of the beef mixture, followed by a layer of salsa and a sprinkle of cheese.
Repeat the layering process: another layer of corn tortillas, the remaining beef mixture, more salsa, and another sprinkle of cheese. Finish with a final layer of corn tortillas topped with the remaining salsa and cheese.
Cover the casserole dish with aluminum foil and bake in the preheated oven for about 25 minutes. Then, remove the foil and bake for an additional 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. Serve with dollops of sour cream and a sprinkle of sliced green onions on top for a fresh finish.
