Start by peeling and slicing the potatoes into thin even pieces. Chop the onion into small pieces. This helps everything cook evenly and blend smoothly in the casserole.

Heat a large skillet over medium heat. Add ground beef and cook until browned. Break it apart while cooking so it becomes crumbly and evenly cooked throughout.

Add chopped onion to the cooked beef. Stir and cook until softened. Add garlic powder, salt, and black pepper. Mix well so flavors combine into a savory base.

In a bowl, mix cream of mushroom soup, milk, and sour cream if using. Stir until smooth and creamy. This sauce keeps the casserole rich and moist while baking.

Grease a 9x13 baking dish with butter. Add half of the sliced potatoes in an even layer. This forms the soft base for the cheesy hamburger casserole.

Spread half of the cooked beef mixture over the potatoes. Make sure it is even so every bite has meat, potatoes, and rich flavor throughout the dish.

Sprinkle cheddar cheese over the beef layer. Pour half of the creamy sauce on top. This helps everything stay moist, creamy, and full of flavor while baking.

Add another layer of potatoes, beef, cheese, and sauce. Repeat layering until ingredients are used. This builds a thick, hearty casserole with rich, balanced flavor.

Cover the dish with foil and bake at 350°F for 50 minutes. Remove foil and bake 10 more minutes until golden, bubbly, and fully cooked through.

Sprinkle mozzarella cheese on top and bake for 5 more minutes until melted. Let it rest before serving so it sets nicely and slices easily for serving.