First, dice the onion the cook then brown the ground beef. When the ground beef is finished toss the diced onion in the pan a little, just until they soften some. Mix the two after.
Preheat oven to 350℉.
Now, wash, peel and slice the potatoes into a scallop style.
In a small bowl, thin the cream of mushroom out by mixing it with the milk.
In a 16x9 baking dish, add some sauce to the bottom then layer the potatoes and the ground beef. Cover the baking dish with foil.
Then, place in the pre-heated oven for 60-90 minutes depending on when the potatoes are all cooked and fork tender.
Uncover the baking dish then cover the casserole with shredded cheese when the potatoes are cook. Place back into the oven for 15 minutes or until cheese is melted.
Allow to cool before serving.