Preheat your oven to 350°F (175°C) and lightly grease a 9×13-inch casserole dish with olive oil or cooking spray.

In a large skillet, heat 1 tbsp olive oil over medium heat. Add the ground beef and cook until browned, about 6–8 minutes, breaking it into small crumbles. Drain excess fat and set aside.

Thinly slice 4 medium potatoes and arrange a layer in the bottom of the prepared dish. Sprinkle with ½ tsp salt, ½ tsp pepper, and ½ tsp garlic powder. Repeat until all potatoes are layered.

In a medium bowl, whisk together 1 can cream of mushroom soup and ½ cup milk until smooth. Pour evenly over the potatoes, coating each layer lightly.

Spread the cooked ground beef over the potatoes and sprinkle the chopped onion on top.

Sprinkle 1 cup shredded cheddar cheese evenly over the casserole and cover with aluminum foil.

Bake at 350°F (175°C) for 45 minutes. Remove the foil and bake an additional 15 minutes, or until potatoes are tender and cheese is bubbly and golden.

Let the casserole rest 5–10 minutes before slicing to allow the layers to set.