In a skillet, cook the ground beef and onion over medium heat until the beef is browned. Drain any excess fat.
In a large bowl, combine the cream of mushroom soup and milk. Mix well.
In a baking dish, layer half of the sliced potatoes, then half of the ground beef mixture. Pour half of the soup mixture over the beef. Repeat the layers.
Cover the dish with aluminum foil and bake for 1 hour, or until the potatoes are tender.
Remove the foil, sprinkle the shredded cheddar cheese on top, and bake for an additional 10 minutes, or until the cheese is melted and bubbly.
Let the casserole cool for a few minutes before serving. Enjoy!
Notes:
This casserole is perfect for a cozy family dinner.