Preheat oven to 350 degrees, spray casserole dish with non stick cooking spray, set aside.
Dice onions and thinly sliced the potatoes - about ¼ inch thick. (No need to peel the potatoes unless you want to!)
Heat a large skillet over medium-high heat. Add ground beef and diced onions. Cook until browned and no longer pink 5-7 minutes. Drain well.
Add the condensed soup, evaporated milk, cream cheese, and Montreal steak seasoning to the ground beef mixture. Stir until the sauce is smooth and combined.
Add a ladle of sauce to the bottom of the prepared casserole dish. Place a layer of sliced potatoes. Top with ⅓ of the potatoes. Add ⅓ of the remaining sauce and ⅓ of shredded cheese. Repeat layers. Finishing with cheesy beefy sauce. Cover tightly with foil.
Bake in the preheated oven until potatoes are tender, about 1 ¼ to 1 ½ hours.
Remove the aluminum foil. Use a fork to check if the potatoes are tender. Top with remaining shredded cheese, and place under broiler for 1-2 minutes until the cheese is browned. Watch it carefully so it doesn't burn!
Allow to sit loosely covered with foil for 10 minutes before serving.