Preheat the oven to 375 degrees. Spray a 9×13 inch baking dish with cooking spray.
Heat a skillet to medium high heat. Add ground beef, garlic, and onion and cook while breaking the meat apart with a spoon or spatula until the ground beef is completely browned. Drain off any excess fat. Add taco seasoning mix, green chilies, frozen corn, and black beans to the ground beef. Cook until heated through.
In a large bowl combine ground beef mixture, ¾ of the Mexican cheese blend, and all of the tater tots. Stir well to combine.
Pour about 1/3 of the enchilada sauce into the bottom of the prepared baking dish. Add the tater tot mixture to the baking dish and lightly pat the mixture down into a solid, even layer. Pour the remaining enchilada sauce over the tater tots.
Place into the oven and bake at 375 degrees for 40 minutes. During the last few minutes of baking, top the casserole with the remaining Mexican cheese blend and the sliced black olives. Return to the oven and bake until the cheese is melted and bubbly.