Preheat your oven to 375°F (190°C).
Grease a 9x13-inch baking dish with butter.
In a large skillet over medium heat, cook the ground beef until browned, breaking it apart as it cooks (about 5–6 minutes).
Drain excess grease if needed.
Stir in the diced onion, garlic, salt, pepper, and garlic powder. Cook for another 2 minutes, then remove from heat.
Arrange half of the sliced potatoes in an even layer at the bottom of the baking dish.
Spread half of the ground beef mixture over the potatoes.
Repeat with the remaining potatoes and beef.
In a small bowl, mix the cream of mushroom soup with the milk until smooth.
Pour the mixture evenly over the casserole.
Cover the dish with aluminum foil and bake for 40 minutes.
Remove the foil, sprinkle the shredded cheddar cheese over the top, and bake uncovered for another 10 minutes, until the cheese is melted and bubbly.
Let the casserole rest for 5 minutes before slicing. Enjoy warm!