1. Preheat oven to 375°F (190°C) and grease a medium baking dish.
2. In a skillet, cook the ground beef over medium heat until browned. Drain any excess fat.
3. Layer half of the potato slices on the bottom of the baking dish, followed by half the onions, then half the cooked beef.
4. Repeat with the remaining potatoes, onions, and beef.
5. In a small bowl, whisk together the cream of mushroom soup and milk until smooth. Pour evenly over the layers.
6. Cover tightly with aluminum foil and bake for 45 minutes.
7. Remove foil, sprinkle cheese evenly on top, and bake uncovered for another 10–15 minutes until golden and bubbly.
8. Let it rest for 10 minutes before serving.