Preheat your oven to 350 degrees. Coat 2-qt baking dish with cooking spray. Heat dutch oven pan over medium heat. Add beef, onion, and then,  fajita dry mix, after it browns. Add tomatoes with green chilies and juices, taco sauce and 1/2 cup water. Bring to a boil. Stir in beans and Mexicorn. Remove from heat. Spread 2 cups beef mixture in bottom of baking dish.Arrange half of tortilla pieces on top to cover. Sprinkle with 1/2 cup cheese. Spread 1/2 of remaining beef mixture again to layer. Top with remaining torn up large pieces of tortillas. Sprinkle again with 1/2 cup of cheese. Top with remaining beef mixture and top off with last of shredded cheese. Bake until hot and bubbly, usually around 45 minutes or a little more .Let cool a little before you eat! I like to eat by addind some sour cream on top and diced up tomatoes. I get lots of raves on this casserole. Very Good!