Instructions:
Preheat the oven to 350°F (175°C).
In a large skillet, cook the ground beef and chopped onion over medium heat until the beef is browned and the onion is tender. Drain any excess fat.
Grease a 9×13 inch baking dish with butter. Layer half of the thinly sliced potatoes in the bottom of the dish.
Spread the browned beef and onion mixture evenly over the potatoes.
In a small bowl, mix together the cream of mushroom soup, milk, salt, and pepper. Pour half of this mixture over the beef and potatoes.
Sprinkle 1 cup of shredded cheddar cheese over the top.
Layer the remaining potatoes over the cheese, then pour the remaining soup mixture on top.
Cover the dish with foil and bake in the preheated oven for 30 minutes.
Remove the foil, sprinkle the remaining 1 cup of cheese on top, and bake for an additional 15-20 minutes, or until the potatoes are tender and the cheese is golden and bubbly.
Let the casserole cool for a few minutes before serving to allow the layers to set.
