In a large skillet, heat a drizzle of oil over medium heat. Add the ground beef and chopped onion, cooking for about 7-10 minutes until the meat is browned and no longer pink. Stir occasionally, breaking up the beef with a spatula, until the onion becomes translucent.

While the beef cooks, wash and thinly slice the potatoes to about 1/8-inch thick. Layer the sliced potatoes on a cutting board, and prepare your crockpot for assembly.

In the bottom of the crockpot, create a layer with half the sliced potatoes. Follow this with half of the cooked ground beef mixture and a sprinkle of cheddar cheese. Repeat the layers, finishing with the remaining potatoes, beef, and cheese.

In a mixing bowl, whisk together the cream of mushroom soup, milk, garlic powder, salt, and pepper until smooth. Pour this creamy sauce evenly over the layered ingredients in the crockpot.

Cover the crockpot and set it to cook on low for 6 to 8 hours. You'll know it's ready when the potatoes are soft and easily pierced with a fork.

In the last 15 minutes of cooking, remove the lid and sprinkle the remaining cheddar cheese on top. Cover again to allow the cheese to melt.

Once cooked, carefully serve the casserole hot, letting it cool for a few minutes before digging in. If desired, garnish with fresh herbs like parsley or thyme.