Preheat your oven to 350°F (175°C) and grease the baking dish.

In a skillet over medium heat, cook the ground beef and diced onion until browned and translucent, about 7-10 minutes. Drain excess grease.

In a mixing bowl, combine the cream of mushroom soup, milk, garlic powder, salt, and pepper until smooth.

Layer half of the sliced potatoes in the baking dish, followed by half of the beef mixture and a third of the cheese. Repeat layers.

Pour the creamy sauce over the layered ingredients, ensuring it seeps through.

Cover with aluminum foil and bake for 60-70 minutes, until potatoes are fork-tender.

Remove foil and sprinkle remaining cheese on top, bake uncovered for an additional 10-15 minutes until cheese is golden.

Let the casserole rest for 10 minutes before slicing and serving warm.