Brown ground beef and onion in a skillet over medium heat. Drain excess grease.
In a large bowl, combine the cooked ground beef mixture, cream of mushroom soup, milk, salt, and pepper.
Layer half of the sliced potatoes in the bottom of a greased 9x13 inch baking dish. Pour half of the ground beef mixture over the potatoes.
Repeat layers with the remaining potatoes and ground beef mixture.
Bake for 30 minutes. Remove from oven and sprinkle with shredded cheddar cheese. Bake for an additional 15 minutes, or until cheese is melted and bubbly.
Let stand for 10 minutes before serving.
Notes
For extra flavor, add a layer of fried onions before baking.