Preheat oven to 375°F (190°C). Grease a 9×13-inch baking dish.
Brown ground beef in a skillet over medium heat with salt, pepper, and garlic powder. Drain excess fat.
In a bowl, mix cream of mushroom soup with milk and paprika until smooth.
Layer half of the potatoes in the baking dish, followed by half of the beef and half of the onions. Spoon some sauce over the top.
Repeat layers with remaining potatoes, beef, onions, and sauce.
Cover tightly with foil and bake for 50–55 minutes, until potatoes are tender.
Uncover, add cheese if using, and bake 10–15 minutes until melted and bubbly.
Let rest 10 minutes before serving.