Preheat the Oven: Preheat your oven to 350°F (175°C).

Prepare the Baking Dish: Grease a 9x13-inch baking dish with non-stick cooking spray.

Cook the Ground Beef and Onion: In a large skillet over medium heat, cook the ground beef and diced onion until the beef is fully browned and the onions are soft (about 8-10 minutes). Drain any excess fat.

Prepare the Potatoes: Peel and slice the potatoes into 1/4-inch thick rounds, ensuring uniform slices for even cooking.

Layer the Casserole: Layer one-third of the potato slices on the bottom of the prepared baking dish. Season with salt and pepper. Spread one-third of the cooked beef and onion mixture over the potatoes, followed by one-third of the shredded cheese. Repeat these layers two more times, finishing with a layer of cheese on top.

Mix the Sauce: In a medium bowl, combine the Cheddar cheese soup, evaporated milk, and regular milk. Stir until smooth and well-blended. Pour the sauce evenly over the layered casserole.

Bake the Casserole: Cover the baking dish with aluminum foil and bake in the preheated oven for 1 hour. Remove the foil and bake for an additional 15 minutes, or until the top is golden and bubbly.

Let It Rest and Serve: Allow the casserole to rest for 5-10 minutes before serving. This resting time helps the layers set, making it easier to slice and serve.