Preheat oven to 375 degrees.
In a skillet, cook diced onions with cooking oil for 4-5 minute until softened.
Add ground beef and brown thoroughly.
Add in taco seasonings, rotel tomatoes, diced tomatoes, and corn. Mix well. Turn burner off.
In a large skillet, cook tortillas for 10 seconds each side. (This will prevent tortillas from being mushy in the casserole.)
Lightly spray a 13x9 casserole or baking dish. Place 2 cups of meat mixture into dish and spread evenly.
Place tortillas in a single layer, you may have to cut some in half to fit, completely covering the meat mixture.
Add 2 more cups of meat mixture and spread evenly over the tortilla layer.
Place 1 cup of cheese evenly over meat mixture.
Place another layer of tortillas and the rest of meat mixture.
Cover casserole dish with tin foil and place in oven for 25-30 minutes until heated through. Remove tin foil and place 1 cup of cheese over top of casserole, place back in oven for 5 minutes until cheese is bubbly.
Serve with sour cream, pico, cilantro, and/or olives. Enjoy.