Preheat Oven:
Preheat oven to 350°F (175°C). Grease a 9×13-inch baking dish and set aside.

Cook Ground Beef:
In a large skillet, heat 1 tablespoon olive oil over medium heat. Sauté chopped onion until soft (about 5 minutes).
Add ground beef, cook until browned, breaking it up as it cooks. Drain excess fat. Season with garlic powder, paprika, salt, and pepper.

Prep Potatoes:
While beef is cooking, peel and thinly slice potatoes evenly (a mandolin works great for this!).

Make the Sauce:
In a bowl, mix condensed cream of mushroom soup with milk until smooth.

Layer the Casserole:
In prepared dish, layer half the sliced potatoes, half the beef mixture, half the creamy sauce, and 1 cup shredded cheese.
Repeat layers with remaining ingredients.

Bake Covered:
Cover dish with foil and bake for 1 hour.

Bake Uncovered:
Remove foil and bake another 30 minutes, until top is golden and potatoes are fork-tender.

Serve & Garnish:
Let casserole sit for a few minutes. Garnish with chopped parsley and serve warm.