Preheat your oven to 350ºF.
Add the olive oil to a large skillet and add the chopped onions. Cook for 5 minutes over medium heat, stirring occasionally, until the onions are browned.
1 Tablespoon olive oil, 1 white onion, chopped
Add the ground beef to the pan and cook until the beef is no longer pink, about 6-7 minutes. Break up the ground beef into small crumbles as it cooks.
1 pound ground beef
Drain the grease from the pan and discard.
Stir in the condensed soup, milk, salt and pepper then remove the pan from the heat.
1 can (10.5 ounces) condensed cream of mushroom soup, 3/4 cup milk, 1/2 teaspoon salt, 1/4 teaspoon ground black pepper
Scoop about ½ cup of the soup mix into a 9x13 baking dish and spread it across the bottom of the pan. Then, add a layer of sliced potatoes. Alternate between a layer of soup mix and potatoes until your pan is full.
3 medium potatoes, peeled and thinly sliced
Bake the casserole for one hour then remove the dish from the oven and sprinkle with the cheese. Place the dish back in the oven and bake for another 20 minutes. The cheese will be melted and the casserole will be bubbling around the edges.
1 cup shredded cheddar cheese
Let cool for about 15 minutes then serve!