Prep the ingredients by peeling and slicing the potatoes and chopping the onion.
In a large skillet, heat olive oil over medium heat, sauté the onion for 3-4 minutes, then add ground beef and cook until browned (6-8 minutes).
Layer half of the sliced potatoes in a greased baking dish, followed by half of the beef mixture, half of the cream of mushroom soup mixed with milk, and 1 cup of cheese. Repeat layers.
Cover with foil and bake at 350°F for 45 minutes, then uncover and bake for an additional 15-20 minutes until cheese is bubbly and golden.
Let the casserole cool for 10 minutes before serving.