Preheat your oven to 350°F (175°C). Grease the 9x13 inch baking dish with butter or cooking spray.
Peel and slice the potatoes into thin rounds, about 1/8 inch thick.
In a saucepan over medium heat, melt the butter. Once melted, add the milk, sour cream, garlic powder, onion powder, salt, and pepper. Whisk together until well combined and heated through.
In the greased baking dish, layer half of the sliced potatoes evenly. Pour half of the creamy mixture over the potatoes, then sprinkle half of the cheddar and Parmesan cheeses.
Repeat the layering process with the remaining potatoes, creamy mixture, and cheeses.
Cover the baking dish tightly with aluminum foil and bake in the preheated oven for 30 minutes.
After 30 minutes, remove the foil and bake for an additional 15 minutes, or until the top is golden brown and bubbly.
Once done, remove from the oven and let it sit for about 5 minutes. Garnish with chopped green onions before serving.