Preheat Oven: Begin by preheating your oven to 350°F (175°C) and prepare a 9×13 inch baking dish by spraying it with non-stick cooking spray.

Cook Beef and Onion: In a large skillet, cook the diced onion and ground beef until the meat is thoroughly browned and cooked through.

Layer Potatoes: Arrange a layer of sliced potatoes, about ¼ inch thick, at the bottom of the baking dish. Season with salt and pepper.

Add Meat and Cheese: Top the potato layer with about ⅓ of the cooked meat mixture, spreading it evenly. Follow with a layer of ⅓ of the shredded cheddar cheese.

Repeat Layers: Continue layering potatoes, meat, and cheese, ending with a final layer of cheese on top.

Combine Soup and Milk: In a mixing bowl, whisk together the Cheddar cheese soup mix, evaporated milk (or whole milk), and an additional ½ cup of milk. Season with more salt and pepper if desired.

Bake: Pour the soup and milk mixture over the layered casserole. Cover the dish tightly with aluminum foil and bake in the preheated oven for 1 hour.

Brown the Cheese: Remove the foil and return the casserole to the oven for another 15 minutes, or until the cheese is golden and bubbly.

Serve: Check that the potatoes are tender; if not, bake a little longer. Once done, let the casserole cool slightly before serving.