Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish with butter.
In a large mixing bowl, combine the sour cream, milk, garlic powder, onion powder, salt, and pepper. Mix well until smooth.
Layer half of the sliced potatoes in the bottom of the prepared baking dish.
Pour half of the sour cream mixture over the potatoes, then sprinkle with half of the cheddar cheese and half of the green onions.
Repeat the layers with the remaining potatoes, sour cream mixture, cheddar cheese, and green onions.
Finish by sprinkling the grated Parmesan cheese on top.
Cover the baking dish with aluminum foil and bake in the preheated oven for 45 minutes.
Remove the foil and bake for an additional 15-20 minutes, or until the potatoes are tender and the top is golden brown.
Allow to cool for a few minutes before serving.