Prepare the Base

1. Preheat Oven: Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with butter.
2. Sauté Onion and Garlic: In a medium saucepan over medium heat, melt the butter and add the chopped onion. Cook until translucent, about 5 minutes. Add the minced garlic and sauté for an additional minute.

Make the Cream Sauce

1. Add Cream and Milk: Pour the heavy cream and whole milk into the saucepan with the onions and garlic. Stir to combine.
2. Season: Add salt, pepper, and thyme. Bring the mixture to a gentle simmer and remove from heat.
3. Stir in Cheese: Gradually stir in 1 ½ cups of the shredded Gruyère (or cheddar) until melted and smooth.

Assemble the Dish

1. Layer Potatoes: Arrange half of the sliced potatoes in an overlapping layer in the prepared baking dish.
2. Pour Half of the Sauce: Pour half of the cream sauce over the potatoes.
3. Add Remaining Potatoes: Layer the remaining potato slices on top and pour the rest of the sauce over them.
4. Top with Cheese: Sprinkle the remaining ½ cup of Gruyère and the Parmesan cheese on top.

Bake

1. Cover and Bake: Cover the dish with aluminum foil and bake for 45 minutes.
2. Uncover and Brown: Remove the foil and bake for an additional 25 minutes, or until the top is golden brown and bubbly.

Serve

1. Cool Slightly: Let the dish rest for 10 minutes before serving to allow the sauce to set.
2. Garnish (Optional): Sprinkle with fresh thyme leaves or chopped parsley for a festive touch.