Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish to prevent sticking.

In a large skillet over medium-high heat, crumble and cook the ground beef until browned. Add the diced onion and cook until onions are soft and translucent, about 5-7 minutes. Drain excess fat.

Return beef and onions to skillet. Stir in minced garlic, dried thyme, Worcestershire sauce, salt, and pepper. Cook for about 1 minute until fragrant.

Scrub or peel russet potatoes. Slice thinly (about 1/8-inch thick) using a mandolin or sharp knife for even cooking.

In a medium bowl, whisk together cream of mushroom soup, milk, and beef broth until smooth.

Spread seasoned ground beef mixture evenly in the prepared baking dish. Arrange sliced potatoes over the beef.

Pour the cream soup mixture evenly over the potatoes and beef, ensuring coating.

Cover tightly with aluminum foil. Bake for 60 minutes until potatoes are fork-tender.

Sprinkle shredded cheddar cheese over top. Bake uncovered for 10-15 minutes until cheese is melted and bubbly.

Let rest for 10-15 minutes before serving for sauce to thicken.