Preheat the oven to 350°F (175°C).
In a large skillet, heat olive oil over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes.
Add the ground beef to the skillet, breaking it apart with a spoon. Cook until browned and fully cooked, about 5-7 minutes. Drain excess fat if needed.
In a bowl, combine the cream of mushroom soup, milk, garlic powder, salt, and pepper. Mix well.
In a greased baking dish, layer half of the sliced potatoes at the bottom. Top with half of the ground beef mixture, followed by half of the cheese. Repeat the layers with the remaining potatoes, beef, and cheese.
Pour the soup mixture evenly over the casserole.
Cover the dish with aluminum foil and bake in the preheated oven for 45 minutes. Remove the foil and bake for an additional 15 minutes, or until the potatoes are tender and the cheese is bubbly.
Allow to cool for a few minutes before serving. Garnish with fresh parsley if desired.