If using the onion, stir it into the hot beef and cook for 2–3 minutes, just until it softens slightly. (If you prefer, you can skip this step and add the onion raw to the slow cooker for a bit more texture.)
Spray the inside of a 4- to 6-quart slow cooker with cooking spray or lightly grease it with a bit of oil. Spoon 2–3 tablespoons of enchilada sauce over the bottom and spread it around to keep the tortillas from sticking.
Enchilada sauce spread in the bottom of a slow cooker
Enchilada sauce spread in the bottom of a slow cooker
Place a layer of corn tortillas over the sauce, tearing them as needed to cover the bottom in a mostly even layer, like you would for lasagna.
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.
Layered enchilada stack before cooking in the slow cooker
Layered enchilada stack before cooking in the slow cooker
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).
