Preheat your oven to 375°F. Spray a 9×13-inch metal or glass baking dish generously with non-stick cooking spray and set it aside.

Heat a large skillet over medium-high heat. Drop in the ground beef and finely diced yellow onion. Cook for 6 to 8 minutes, using a firm wooden spoon to continuously break the beef apart into small, uniform crumbles, until the meat is thoroughly browned and the onions are soft and translucent.

Stir in the minced garlic and Italian seasoning. Season with a generous pinch of salt and black pepper, and sauté for exactly 60 seconds until highly fragrant. Tilt the skillet to thoroughly drain away any heavy pooling grease, then set the meat aside.

In a medium mixing bowl, combine the condensed cream of mushroom soup, whole milk, sour cream, and a bit more black pepper. Whisk vigorously until the wet sauce baseline is completely smooth and free of lumps.

Use a mandoline or a very sharp chef's knife to slice the peeled potatoes into uniform discs no thicker than ⅛ to ¼ inch. Keeping the slices thin and even is critical to ensure they cook through at the same rate and don't end up crunchy.

Spread a thin spoonful of the soup mixture across the bottom of the prepared baking dish to prevent sticking. Arrange one-third of the sliced potato discs in an even, slightly overlapping layer across the base.

Top the potatoes with half of the cooked hamburger mixture, one-third of the cream sauce, and exactly 1 cup of the shredded cheese.

Repeat the pattern for the second layer: add the second third of the potatoes, the remaining half of the beef, another third of the cream sauce, and another cup of cheese.

For the final top layer, shingle the remaining third of the potatoes, pour the last of the cream sauce uniformly over the surface, and cover it completely with the final ½ cup of shredded cheese.

Cover the baking dish tightly with aluminum foil (spray the inside of the foil with non-stick spray so the melting cheese doesn't stick to it). Bake at 375°F for 45 to 50 minutes.

Carefully remove the foil wrapper. The casserole should be bubbling aggressively around the edges. Return the dish uncovered to the oven for an additional 15 to 20 minutes, or until the top cheese layer is entirely melted, blistered, and takes on beautiful golden-brown caramelized patches.

Insert a fork into the center of the dish; the prongs should slide effortlessly through the potato layers. Remove the casserole from the oven and let it rest flat on a wire rack for 10 minutes to allow the internal emulsions to tighten before slicing into clean, neat portions. Garnish with parsley and serve hot.