Preheat your oven to 375°F (190°C).
Grease a large baking dish with butter.
Layer half of the sliced potatoes in the bottom of the dish.
In a saucepan, melt the butter over medium heat and sauté the chopped onion and minced garlic until fragrant and translucent, about 5 minutes.
Add the heavy cream, cheddar cheese, Parmesan cheese, salt, and pepper to the saucepan. Stir continuously until the cheese is melted and the mixture is smooth.
Pour half of the cream mixture over the potatoes in the baking dish.
Layer the remaining potato slices on top and pour the rest of the cream mixture over them.
Cover the dish with aluminum foil and bake for 45 minutes. Then, remove the foil and bake for an additional 20-25 minutes, or until the top is golden brown and the potatoes are tender.
Sprinkle with chopped parsley before serving, if desired.
