Preheat oven to 375°.
In a skillet, brown ground beef and minced onion. Drain grease and place to the side.
Peel and slice potatoes. Spray casserole dish with cooking spray. Place potato slices in a single layer on the bottom of the dish.
Top potato layer with 1/2 the ground beef and 1/3 of the cheese.
Repeat the potato and ground beef/cheese layer once more.
Add a third layer of potato slices.
In a small mixing bowl, combine the milk and mushroom soup. Whisk to combine.
Pour soup mixture over casserole, then top with remaining cheese.
Cover tightly with foil, then bake at 375° for 45 minutes.
Remove foil and bake for additional 15 minutes.
Allow casserole to rest about 10 minutes before serving.