Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish with nonstick spray or butter.

In a large skillet over medium-high heat, cook the ground beef and diced onion together, breaking up the beef with a wooden spoon as it cooks, until the beef is fully browned and the onions are translucent, about 8-10 minutes. Drain any excess grease.

Season the cooked beef mixture with garlic powder, thyme, salt, and pepper while still hot.

In a medium bowl, whisk together both cans of condensed soup and the milk until completely smooth with no lumps.

Arrange half of the thinly sliced potatoes in an even layer across the bottom of the greased baking dish, overlapping slightly. Lightly season with salt and pepper.

Spread half of the cooked ground beef mixture evenly over the potatoes. Pour half of the soup mixture over the beef layer. Sprinkle 1 cup of the shredded cheddar cheese over the sauce.

Repeat the layers with the remaining potato slices, beef, and soup mixture.

Cover the dish tightly with aluminum foil. Bake for 45-50 minutes, or until the potatoes are fork-tender.

Remove the foil. Sprinkle the remaining 1 cup of cheese evenly over the top, then scatter the crispy fried onions over the cheese. Bake uncovered for another 10-15 minutes until the cheese is melted and bubbling and the fried onions are golden brown.

Let the casserole rest for 10 minutes before serving to allow the sauce to set up.