Preheat & Prep: Preheat your oven to 350°F (175°C). Generously spray a springform pan, round cake pan, or deep casserole dish with non-stick cooking spray.
Cook the Meat: In a large skillet over medium heat, brown the ground beef and diced onion until the meat is fully cooked. Drain excess grease. Add the taco seasoning and water according to the packet instructions. Simmer until thickened.
Layer 1: Place 1 flour tortilla in the bottom of the prepared baking dish. Spread 1/3 of the warmed refried beans over the tortilla. Top with 1/3 of the meat mixture, and sprinkle with a layer of cheddar cheese.
Layer 2: Place a 2nd tortilla on top. Spread 1/2 of the prepared rice, 1/2 of the salsa, and all of the drained corn. Top with cheese.
Layer 3: Add a 3rd tortilla. Spread another 1/3 of the beans, 1/3 of the meat, and sprinkle with cheese.
Layer 4: Add the 4th tortilla. Top with the remaining beans, meat, rice, and salsa. Cover the entire top with the remaining shredded cheddar cheese. (If using a 5th tortilla, place it down before this final topping layer).
Bake: Cover the dish tightly with aluminum foil. Bake at 350°F for 40 minutes.
Brown the Cheese: Remove the foil and bake for an additional 10-15 minutes, or until the cheese is completely melted and slightly bubbly.
Rest & Serve: Remove from the oven and let the casserole rest for at least 5 minutes before slicing. Garnish with sour cream, fresh lettuce, and diced onions if desired. Serve hot and enjoy!