Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.

In a large skillet over medium-high heat, brown the ground beef with the diced onion, breaking the meat into small crumbles as it cooks. Once the beef is no longer pink, add the minced garlic and cook for 1 more minute. Drain any excess grease thoroughly.

In a medium bowl, whisk together the condensed cream of mushroom soup, whole milk, sour cream, Italian seasoning, salt, and pepper until smooth and evenly combined.

Spread about 1/4 cup of the soup mixture across the bottom of the greased baking dish.

Arrange half of the sliced potatoes in an overlapping, even layer over the sauce. Top with half of the cooked beef mixture, then pour half of the remaining sauce evenly over the beef. Sprinkle with 1 cup of the shredded cheese.

Repeat with the remaining potatoes, the rest of the beef, and the rest of the sauce. Reserve the final cup of cheese for the uncovered bake.

Cover the dish tightly with aluminum foil, pressing the edges against the rim to create a proper seal. Bake for 60 minutes.

Remove the foil carefully and scatter the remaining cup of cheese evenly over the top. Return the dish to the oven uncovered for 15 minutes, until the cheese is bubbly and beginning to turn golden.

Let the casserole rest for 10 minutes before serving.