Prepare the beef: Heat olive oil in a large skillet over medium-high heat. Season the beef with salt and pepper, then sear it in the hot skillet for about 4-5 minutes per side until browned. Once browned, transfer the beef to your Crock Pot.

Cook the beef: In the same skillet, sauté the chopped onion and garlic for 2-3 minutes until softened. Add the enchilada sauce, diced green chilies, taco seasoning, and beef broth. Stir to combine and bring to a simmer for 2-3 minutes.

Add to the Crock Pot: Pour the sauce mixture over the beef in the Crock Pot. Cover and cook on low for 6-7 hours or on high for 3-4 hours, or until the beef is fork-tender and can be easily shredded with two forks.

Shred the beef: Once the beef is done cooking, remove it from the Crock Pot and shred it with two forks. Return the shredded beef to the Crock Pot and stir it into the sauce.

Assemble the casserole: Preheat the oven to 375°F (190°C). In a 9×13-inch baking dish, spread a thin layer of the shredded beef mixture on the bottom. Layer with tortillas (cut them in halves or quarters to fit), then add more beef mixture, followed by a layer of cheese. Repeat the layers until all the beef, tortillas, and cheese are used up, finishing with a generous layer of cheese on top.

Bake: Cover the casserole with foil and bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly. For a slightly crispy top, uncover the casserole during the last 5 minutes of baking.

Serve: Remove the casserole from the oven and let it cool for 5 minutes. Top with sour cream, fresh cilantro, and any other toppings of your choice, such as diced tomatoes, jalapeños, or avocado.