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 degrees F. Spray a 9x13 inch baking dish with oil and add 1/2 cup enchilada sauce.
Arrange 3 tortillas on the bottom followed by 1/2 of the meat, about 1/2 cup cheese, 1/2 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 minute before slicing. Garnish with tomatoes, cilantro, and sour cream.