Preheat the oven to 375°F. Lightly coat a 9 × 13-inch baking dish with nonstick cooking spray.
Add the ground beef and diced onion to a large skillet. Cook over medium-high heat for 7–9 minutes, breaking the beef into crumbles, until the meat is browned and the onion is tender.
Drain the excess grease. Stir in the garlic, salt, and black pepper and cook for 30 seconds. Remove the skillet from the heat.
In a medium bowl, whisk together the condensed cream of mushroom soup, milk, Worcestershire sauce, and onion powder until smooth.
Peel the potatoes and cut them into uniform slices approximately ⅛ inch thick. A mandoline slicer works best for creating evenly sized slices.
Arrange half of the sliced potatoes in the prepared baking dish, overlapping them slightly as needed.
Top the potatoes with half of the ground beef mixture, half of the soup mixture, and ¾ cup of the shredded cheddar.
Repeat the layers with the remaining potatoes, ground beef, and soup mixture. Reserve the remaining 1¼ cups of cheese.
Cover the baking dish tightly with aluminum foil. Bake for 60 minutes.
Carefully remove the foil and test several potato slices with a thin knife. If the potatoes are still firm, cover the casserole again and bake for another 10–20 minutes, checking periodically.
Once the potatoes are tender, uncover the casserole and sprinkle the remaining cheese evenly over the top.
Bake uncovered for 10–15 minutes, until the cheese is completely melted and the sauce is bubbling around the edges.
Let the casserole rest for 10–15 minutes before serving. Garnish with sliced green onions, if desired.
