Preheat your oven to 350°F (175°C).

Heat vegetable oil in a skillet over medium heat, then add the lean ground beef and cook until browned, about 8 minutes.

Add the medium onion and minced garlic to the skillet; cook until the onion is tender, around 5 minutes.

Stir in the taco seasoning, black beans, corn, diced tomatoes with green chilis, and salsa; let everything simmer for 5 minutes.

Grease a 9x13-inch baking dish lightly, then layer half of the tortilla quarters on the bottom.

Spread half of the seasoned beef mixture over the tortillas, then sprinkle with 1 cup of shredded cheese.

Layer the remaining tortilla quarters and top them with the rest of the beef mixture and a generous amount of sour cream.

Top off your casserole with the remaining cheese, sliced black olives, and chopped green onions.

Cover the dish with foil and bake for 25 minutes. Then, remove the foil and bake for an additional 10 minutes until the cheese is bubbly.

Cool for 10 minutes before serving to let the flavors settle.