Preheat your oven to 375°F (190°C) to prepare for baking.
Peel and slice the potatoes thinly. Keep them in a bowl of cold water to prevent browning.
In a large skillet, brown the ground beef with the chopped onion. Season with garlic powder, salt, and pepper.
Layer half of the potatoes, beef mixture, and cheese in a 9x13 inch baking dish. Repeat the layers.
Mix milk and melted butter, pour over the casserole, and cover with foil.
Bake covered for 45 minutes. Uncover and bake for an additional 15 minutes.
Notes
Store leftover casserole in an airtight container in the refrigerator for up to 3 days or freeze portions for longer keeping. Remember to thaw before reheating.