Preheat your oven to 375°F (190°C).
In a large saucepan over medium heat, melt the butter. Once melted, whisk in the heavy cream and milk until smooth.
Stir in garlic powder, onion powder, salt, and pepper to taste. Allow the mixture to come to a gentle simmer.
In a large mixing bowl, combine the sliced potatoes with one cup of shredded cheddar cheese and half of the Parmesan cheese.
Pour the cream mixture over the potato and cheese mixture, ensuring all potatoes are well coated.
Transfer the potato mixture into a greased 9x13 inch baking dish. Spread evenly.
Cover the dish with aluminum foil and bake for 30 minutes.
Remove the foil, sprinkle the remaining cheese on top, and bake for an additional 15 minutes, or until the potatoes are tender and the cheese is golden and bubbly.
Remove from the oven and let it sit for about 5 minutes. Garnish with fresh parsley, if desired, before serving.