Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

In a large skillet, brown the ground beef over medium-high heat. Drain off any excess grease.

Add the chopped onion to the skillet with the beef and cook until softened, about 5-7 minutes.

Season the beef and onion mixture with salt and pepper to taste.

In a medium bowl, whisk together the condensed cream of mushroom soup and milk until smooth.

Arrange half of the thinly sliced potatoes in an even layer at the bottom of the prepared baking dish.

Spread the beef and onion mixture evenly over the layer of potatoes.

Pour half of the soup mixture over the beef and onion.

Arrange the remaining half of the thinly sliced potatoes over the soup mixture.

Pour the remaining soup mixture evenly over the top layer of potatoes.

Cover the baking dish tightly with aluminum foil.

Bake for 60 minutes. Remove the foil and, if desired, sprinkle with shredded cheddar cheese.

Bake uncovered for an additional 15-20 minutes, or until potatoes are tender and cheese is melted and bubbly.

Let stand for 5-10 minutes before serving.