Preheat oven to 375°F and grease a 9x13-inch baking dish.
Brown ground beef with diced onion in a large skillet over medium-high heat, breaking up meat as it cooks, about 8 minutes.
Drain excess fat and season beef with garlic powder, paprika, salt, and pepper.
In a bowl, whisk together cream of mushroom soup and milk until smooth.
Layer half the sliced potatoes in the bottom of the baking dish, overlapping slightly.
Spread the cooked ground beef mixture over the potatoes, then pour half the soup mixture on top.
Sprinkle with 1 cup of cheese, then layer remaining potatoes on top.
Pour remaining soup mixture over potatoes, dot with butter pieces, cover with foil, and bake for 45 minutes, then uncover, top with remaining cheese, and bake 15 minutes more until potatoes are tender and cheese is golden.