In a large skillet, heat a splash of oil over medium heat. Add the ground beef and chopped onion, cooking for about 7-10 minutes until browned and onion is translucent. Drain excess grease and set aside.

Wash and dry the potatoes thoroughly. Slice them into uniform rounds about 1/8-inch thick using a sharp knife or mandoline.

Layer half of the potato slices in the crockpot. Follow with half the cooked beef mixture and sprinkle cheddar cheese. Repeat layers with remaining potatoes, beef, and cheese.

In a medium bowl, combine cream of mushroom soup, whole milk, garlic powder, salt, and pepper. Whisk until smooth, about 2-3 minutes.

Pour the sauce mixture evenly over layered ingredients in the crockpot, ensuring it covers everything.

Cover the crockpot and cook on low for 6-8 hours until potatoes are tender and cheese is melted.

In the last 15 minutes, remove the lid and sprinkle remaining cheddar cheese on top. Close lid and cook until cheese is melted and bubbly.