Preheat oven to 375°F (190°C).

In a large skillet over medium heat, cook the ground beef and diced onion until fully browned. Drain excess grease.

Season the beef mixture with salt, pepper, and garlic powder. Stir well and set aside.

In a small bowl, mix the cream of mushroom soup with milk until smooth.

Lightly grease a 9x13-inch baking dish.

Layer half of the sliced potatoes evenly in the bottom of the dish.

Spread half of the cooked ground beef mixture over the potatoes.

Pour half of the soup mixture evenly over the beef layer.

Sprinkle one cup of shredded cheddar cheese over the top.

Repeat the layers with the remaining potatoes, beef, soup mixture, and finish with the remaining cheese.

Cover the baking dish tightly with aluminum foil.

Bake for 45 minutes.

Remove the foil and bake an additional 15 minutes, or until the potatoes are tender and the cheese is golden and bubbly.

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

Garnish with chopped parsley if desired.