Preheat the oven to 375 degrees. Heat a large skillet over medium-high heat and add the oil. Once the oil is hot, add the tortillas and cook on both sides to soften the tortillas. Remove the tortillas from the pan and set them aside.
12-14 corn or flour tortillas, 1 tablespoon oil
Use the same skillet over medium-high heat and add the ground beef and onions. Cook for 5-6 minutes, breaking up the meat as it cooks, until the beef is browned and the onions are softened. Drain any excess fat if needed.
1 pound ground beef, ½ cup diced onions
Add the garlic and cook for 30 seconds until fragrant. Sprinkle in the taco seasoning and stir well to coat the beef. Add the diced green chiles and corn. Stir to combine and simmer for 3-5 minutes until heated through. Taste the mixture and adjust the spices and seasoning as needed.
4 garlic cloves, taco seasoning, 7 ounce canned diced green chiles, 1-2 cups corn
Spread a thin layer of enchilada sauce into the bottom of a 9x13-inch baking dish. Arrange a layer of tortillas over the top, cut or tear them as needed to fit. Spoon a portion of the beef mixture over the tortillas, drizzle some enchilada sauce, and sprinkle with a layer of cheddar and Monterey cheese.
2 cups enchilada sauce, 3 cups grated cheddar and Monterey jack cheese
Continue layering tortillas, beef mixture, enchilada sauce, and cheese until all of the ingredients have been used. Finish with a generous layer of cheese on top.
Cover the baking dish with foil and bake for 20 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is melted, bubbly, and lightly golden around the edges.
Allow the casserole to rest for 10 minutes before slicing. This helps the layers set and makes serving much easier. Garnish with your favorite toppings and serve.