Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish.
Step 2: In a large skillet over medium heat, cook ground beef, breaking it up, until browned. Drain off any excess grease.
Step 3: Return the skillet with the drained beef to the heat. Add diced onion, garlic powder, salt, and pepper. Cook for 2-3 minutes until the onion softens slightly and is fragrant.
Step 4: In a very large bowl, combine the cooked ground beef mixture, thinly sliced raw potatoes, cream of mushroom soup, milk, and sour cream. Stir in about half of the shredded cheddar cheese until everything is well combined and the potatoes are coated.
Step 5: Pour the mixture evenly into the prepared 9x13 inch baking dish. Spread it out and sprinkle the remaining shredded cheddar cheese all over the top.
Step 6: Cover the baking dish snugly with aluminum foil. Bake in the preheated oven for 30 minutes.
Step 7: Carefully remove the foil. Return the casserole to the oven and bake for another 15-20 minutes, or until the cheese is melted, bubbly, and golden brown, and the potatoes are tender when poked with a fork.
Step 8: Take the casserole out of the oven and let it rest for about 5-10 minutes before serving. This helps it set up slightly.