Spread about one-third of the beef (and onion, if using) over the tortillas. Drizzle with about one-third of the remaining enchilada sauce and sprinkle with one-third of the cheese.
Repeat the layers—tortillas, beef, sauce, cheese—two more times, ending with a generous layer of cheese on top. Don’t worry if it’s not perfect; it all melds together as it cooks.
Cover the slow cooker with the lid and cook on LOW for 3–4 hours or on HIGH for about 2 hours, until everything is hot and bubbly and the cheese has melted nicely.
Turn off the slow cooker and let the enchilada stack rest, covered, for about 10–15 minutes so it can set up a bit. Then cut into wedges or scoop out with a large spoon. Serve warm, with any favorite toppings on the side if you like (sour cream, sliced green onion, or a bit of chopped tomato).
