Preheat oven to 400 degrees F and grease a 9x13 baking dish.
In a large skillet over medium heat, cook meat and onion until meat is browned and cooked completely through.
Stir in salsa, black beans, corn, taco seasoning, cumin, and sour cream to the meat. until evenly mixed through.
Spread 1/3 of meat mixture onto the bottom of the greased 9x13 baking dish.
Sprinkle 2/3-3/4 cup shredded cheese over the top of meat mixture.
Layer with 2 tortillas. If desired, break and trim a third tortilla to fill in any gaps.
Pour 1/3 of the enchilada sauce over the tops of the tortilla layer.
Continue layering as listed above until top layer is reached. End with toping the casserole with tortillas, last 1/3 of enchilada sauce and shredded cheese. Be generous on the cheese :)
Cover with foil and bake at 400 degrees F for 25 minutes or until bubbly and cheese is melted.
Serve with any toppings you would like.