Preheat the oven to 350°F (175°C) and lightly coat a 9x13-inch baking pan with non-stick spray or a thin layer of oil.
In a large skillet set over medium heat, brown the ground beef together with the chopped onion until the meat is fully cooked and no pink remains. Drain off any rendered fat.
In a mixing bowl, stir together the mushroom soup, milk, garlic powder, salt, and pepper until you achieve a uniform consistency.
Arrange a layer of the thinly sliced potatoes across the bottom of the prepared dish. Spoon a portion of the cooked beef mixture over the potatoes. Pour some of the soup mixture over the meat layer, and sprinkle a handful of the shredded cheddar. Repeat the layering process until all ingredients are used, reserving half of the cheese for later.
Cover the baking dish tightly with foil and place in the oven. Bake for 60 to 70 minutes, or until the potatoes are easily pierced with a fork.
Uncover the dish, scatter the remaining cheese evenly on top, and return to the oven for another 10 to 15 minutes, or until the cheese is golden and bubbling.
Once removed from the oven, allow the casserole to sit undisturbed for 10 minutes to set. Garnish with chopped parsley if using, and serve warm.