Preheat the Oven: Preheat your oven to 350°F (177°C). Spray a 9x13-inch baking dish generously with nonstick cooking spray and set aside.

Cook the Meat: In a large skillet over medium-high heat, combine the ground beef and finely diced yellow onion. Season generously with kosher salt and black pepper. Cook until the beef is completely browned and no longer pink, breaking it up into small crumbles as it cooks. Drain any excess fat from the skillet and set the meat mixture aside.

Layer the Ingredients: In the prepared baking dish, place an even layer of the sliced potatoes. Season the potato layer lightly with salt and pepper. Add 1/4 of the cooked beef and onion mixture directly over the potatoes, followed by 1/4 of the grated cheddar cheese. Repeat this exact layering process three more times to ensure completely even distribution.

Prepare the Sauce: In a small mixing bowl, whisk together the condensed cheddar soup, evaporated milk, whole milk, and garlic powder until completely smooth and cohesive.

Bake: Pour the cheese sauce evenly over the layered ingredients in the baking dish. Cover the dish tightly with aluminum foil. Bake in the preheated oven for 60 minutes. After 60 minutes, remove the foil and bake for an additional 15 minutes, or until the top is golden brown and the sauce is bubbling around the edges.

Rest and Serve: Remove the casserole from the oven and let it sit undisturbed for 10 to 15 minutes before serving. This resting period allows the sauce to set properly for easier slicing. Enjoy!