Preheat oven to 375°F (190°C). Grease a 9×13-inch baking dish.
In a skillet over medium heat, cook ground beef and diced onion until browned. Drain excess grease.
In a bowl, whisk together milk, cream soup, salt, pepper, garlic powder, and paprika.
Layer half of the sliced potatoes in the baking dish. Top with half of the cooked beef and half of the cheese.
Repeat layers with remaining potatoes, beef, and cheese.
Pour sauce evenly over the casserole.
Cover with foil and bake for 45 minutes.
Remove foil and bake an additional 15 minutes, until cheese is bubbly and potatoes are tender.
Let rest for 10 minutes before serving.