Preheat your oven to 375°F (190°C).
Heat olive oil in a skillet over medium heat. Cook onions until softened.
Add ground beef, garlic powder, salt, and pepper. Cook until beef is browned and crumbled; drain excess fat.
In a bowl, combine cream of mushroom soup and milk until smooth.
Layer half of the potato slices in a greased casserole dish.
Spread the ground beef mixture evenly over potatoes.
Top with remaining potato slices.
Pour the soup mixture evenly over the casserole and sprinkle with shredded cheddar cheese.
Cover with aluminum foil and bake for 45 minutes.
Remove foil and bake another 15 minutes until potatoes are tender and cheese is bubbly and golden.