Brown ground beef with onions in a large skillet. Drain off any fat. Mix in the taco seasoning, black beans, and frozen corn.
Preheat the oven to 350°F. Spray a 9x13 inch baking dish with oil and add ½ cup enchilada sauce.
Arrange 3 tortillas on the bottom followed by ½ of the meat, about ½ cup cheese, ½ cup enchilada sauce, and then 3 more tortillas. Repeat and then finish with 3 more tortillas, remaining enchilada sauce, and cheese.
Cover and bake for 25 minutes. Remove foil and bake for 5 more minutes until cheese is fully melted and begins to brown.
Let casserole sit 5-10 minutes before slicing. Garnish with tomatoes, cilantro, and sour cream.
