Preheat oven to 350°F (175°C).
Heat a large skillet over medium-high heat. Cook and stir ground beef in the hot skillet until browned and crumbly, 5 to 7 minutes. Drain and discard grease.
In a medium bowl, mix condensed soup, milk, onion, salt, and pepper until well combined.
In a 2-quart baking dish, create alternating layers of potatoes, soup mixture, and browned ground beef.
Cover the baking dish with aluminum foil and bake in the preheated oven for about 1 hour.
Check if the potatoes are tender with a fork. If not, continue baking for another 15-30 minutes until potatoes are completely tender.
Remove foil, sprinkle the top with Cheddar cheese, and return to the oven uncovered for about 5-10 minutes until cheese is melted and bubbly.
Allow casserole to rest for 10 minutes before serving.