Preheat oven
Preheat your oven to 350°F (175°C). Lightly grease a 9×13-inch baking dish with butter or cooking spray.

Cook the beef mixture
In a large skillet over medium heat, cook the lean ground beef until it’s browned and fully cooked, breaking it apart as it cooks. Add the chopped onion and sauté for 2-3 minutes until softened. Season with salt and black pepper to taste. Drain any excess grease.

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

Assemble the casserole
Layer half of the sliced potatoes evenly on the bottom of the prepared baking dish. Spread half of the beef mixture over the potatoes. Pour over half of the soup mixture. Repeat the layers: remaining potatoes, remaining beef, and the rest of the soup mixture.

Top with cheese
Sprinkle the shredded cheddar cheese evenly over the top of the casserole.

Bake
Cover the dish tightly with aluminum foil and bake in the preheated oven for 1 hour. Remove the foil and bake for an additional 15-20 minutes, or until the potatoes are tender and the cheese is melted and bubbly.

Let it rest and serve
Allow the casserole to rest for 5-10 minutes before slicing and serving. This helps it set and makes it easier to serve.