Cook the ground beef and chopped onion in a skillet over medium heat until the beef is browned with no pink remaining, about 5 to 7 minutes. Drain excess fat.
Wash and thinly slice the potatoes about 1/8 inch thick for even cooking.
In the crockpot, layer half of the sliced potatoes first, then half of the cooked beef and onion mixture, followed by a portion of shredded cheddar cheese.
Repeat the layering with the remaining potatoes, beef, and some more shredded cheddar cheese.
In a mixing bowl, whisk together the cream of mushroom soup, milk, garlic powder, salt, and pepper until smooth.
Pour the sauce evenly over the layered ingredients in the crockpot.
Cover and cook on low for 6 to 8 hours, or until the potatoes are tender and the cheese is melted and bubbly.
About 15 minutes before serving, sprinkle the remaining shredded cheddar cheese on top. Cover to melt the cheese.
Garnish with fresh parsley or chopped chives if desired before serving.