Oven Preparation: Preheat the oven to 350°F (177°C). Utilize nonstick spray to coat a 9x13-inch baking dish comprehensively. Set aside.

Meat Sauté: Place ground beef and diced onion into a large skillet over medium-high heat. Season generously with salt and pepper. Cook until the beef is thoroughly browned and no longer pink, ensuring to break up the beef as it cooks. Upon completion, drain any excess fat from the skillet and set the cooked meat aside.

Assembly of Layers: In the prepared baking dish, arrange a single layer of sliced potatoes. Season each layer with salt and pepper. Follow with a layer comprising ¼ of the cooked beef and onion mixture, and then sprinkle ¼ of the grated cheddar cheese over the beef. Repeat this layering process three more times, ensuring even distribution of ingredients.

Sauce Preparation: In a small mixing bowl, combine the condensed cheddar soup, evaporated milk, milk, and garlic powder. Whisk these ingredients together until the mixture is smooth and homogenous.

Baking: Pour the prepared sauce evenly over the assembled layers in the baking dish. Cover the dish tightly with aluminum foil and place it in the preheated oven. Bake for 60 minutes. After this initial baking period, remove the foil and continue to bake for an additional 15 minutes, or until the top layer is visibly golden brown and the sauce is bubbling.

Serving: Remove from oven and allow the casserole to stand for several minutes before serving to ensure that it sets properly and is safe to consume.