Preheat the oven to 375.
Heat the oil in a large skillet over medium heat. Add the onions to the skillet and cook until softened. Add the ground beef to the skillet and cook, breaking up with a wooden spoon, until browned and cooked through. Add the garlic powder, cumin, chili powder, salt, and pepper, and mix until well blended.
Add the black beans, sweet corn, and diced tomatoes to the skillet, and stir to combine. Bring the mixture to a simmer, and simmer 5-10 minutes, then remove from the heat.
Spray a 9 x 9 baking dish with cooking spray. Spread 1/2 cup of the meat mixture in the bottom of the baking dish, then layer four tortillas on top. Spread 1/3 of the remaining meat mixture over the top, and sprinkle with 1/2 cup of cheese.
Layer four more tortillas over the cheese, then spread 1/3 of the meat mixture over the top, and sprinkle with cheese. Repeat the layers one more time (for three layers total), finishing with the remaining 1 cup of cheese.
Bake 25-30 minutes, or until the casserole is heated through and the cheese is melted and bubbling. Serve hot with sour cream and the taco toppings of your choice.