Preheat oven to 350°F (175°C) and grease a baking dish with butter.
In a skillet cook ground beef and diced onion until browned.
Drain excess grease and season with garlic powder, salt, and black pepper.
In a bowl mix cream of mushroom soup and milk until smooth.
Layer half of the sliced potatoes in the baking dish.
Spread half of the beef mixture over the potatoes.
Pour half of the soup mixture over the layer.
Repeat layers with remaining potatoes, beef, and soup mixture.
Sprinkle shredded cheddar cheese evenly on top.
Cover with foil and bake for 35 minutes.
Remove foil and bake another 10 minutes until potatoes are tender and cheese is melted.
