Preheat the oven to 350 degrees F .
Brown the ground beef over medium heat, and be sure to drain the fat.
Combine the cream of mushroom soup, onion, milk, salt & pepper, in a mixing bowl.
Mix until well combined.
Layer the potatoes, soup mixture, and meat in a 2 quart baking dish.
Bake in the oven for 1 hour and 15 minutes, or until potatoes are nice and fork tender.
Remove the casserole from the oven, and top with sharp cheddar cheese , and green onion.
Continue baking until the cheese is melted.
Remove from the oven, and let sit for 5 minutes.
Serve & enjoy!