In a large skillet, heat the vegetable oil over medium heat. Add the chopped onion and sauté until it becomes translucent, about 3-4 minutes. Then, add the ground beef, breaking it apart with your wooden spoon. Cook until browned, about 5-7 minutes. Drain any excess fat.
Stir in the taco seasoning mix and water, mixing until well combined. Let the mixture simmer for about 2-3 minutes, allowing the flavors to meld together.
Preheat your oven to 350°F (175°C). In your prepared casserole dish, start layering your ingredients. Begin with a layer of corn tortillas, followed by half of the beef mixture, half of the salsa, half of the refried beans, and a third of the shredded cheese.
Repeat the layering process: add another layer of corn tortillas, the remaining beef mixture, the rest of the salsa, the remaining refried beans, and another third of the shredded cheese.
Finish with a final layer of corn tortillas on top, then sprinkle with the remaining cheese.
Cover the casserole with foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden.
Once out of the oven, let the casserole cool for a few minutes. Top with sour cream and sliced green onions before serving. Enjoy your Easy Taco Casserole with your favorite sides or alone!