Preheat your oven to 375F (190C) and lightly grease a 9x13 inch baking dish.
In a large skillet over medium heat, brown the ground beef, chopped onion, and minced garlic until the meat is fully cooked. Drain excess fat.
Place half of the sliced potatoes in an even layer at the bottom of the prepared baking dish.
Spread the cooked beef mixture evenly over the potatoes. Season with half of the salt and black pepper.
Whisk the cream of mushroom soup and milk together in a bowl, then pour the mixture over the beef layer.
Layer the remaining sliced potatoes on top. Season with the remaining salt and pepper.
Cover the baking dish tightly with aluminum foil and bake for 35 minutes until the potatoes are tender.
Remove the foil, sprinkle the shredded cheddar cheese evenly over the casserole, and return to the oven uncovered for 10 minutes until the cheese is melted.
Remove from the oven, garnish with fresh parsley, and let rest for 5 minutes before serving.