Preheat oven to 375°F and grease a 9x13-inch baking dish.
Brown ground beef with diced onion over medium-high heat. Season with garlic powder, paprika, salt, and pepper. Drain excess fat.
Whisk together cream of mushroom soup and milk until smooth.
Layer half the sliced potatoes in the baking dish. Top with half the beef mixture, then spoon half the soup mixture over it. Repeat layers.
Sprinkle cheddar cheese evenly over the top.
Cover tightly with foil and bake for 45-50 minutes.
Remove foil and bake 15-20 more minutes until cheese is golden and bubbly and potatoes are fork-tender.
Let rest 5-10 minutes before serving. Garnish with fresh parsley.
Notes
For even cooking, slice potatoes uniformly thin (about 1/8 inch). You can swap cheddar for Monterey Jack or pepper jack. This casserole freezes well for up to 3 months — assemble unbaked and freeze, then add 10 minutes to baking time.