Preheat the oven to 400 degrees F. Spray an 8x11 or 9x9 inch baking dish (or a similar sized pan) with cooking spray.
Heat the olive oil in a large skillet over medium low heat and add the onion, garlic, cumin and chili powder. Stir for a few minutes until the onions are soft and the spices are fragrant, about 3-5 minutes. Add the tomatoes, reserved tomato juice, tomato paste, beans, corn and spinach. Stir and cook for another minute or two until the spinach has wilted and the corn has thawed. Season with salt and pepper to taste.
Place two corn tortillas in the bottom of the prepared pan and top with 1/4 of the bean mixture, spreading the mixture evenly. Sprinkle evenly with 1/2 cup of the cheese. Repeat until all of the ingredients have been used up, finishing with the cheese.
Bake until the tortilla casserole has cooked through, approximately 20 minutes. Allow to cool for 5 minutes before cutting.
Serve with garnishes, if desired (I highly recommend them!).