Preheat your oven to 400°F (200°C) and grease a baking dish with butter or non-stick spray.

Peel and thinly slice the Yukon gold potatoes about 1/8 inch thick. Place in a bowl of cold water to prevent browning.

Melt 4 tablespoons of unsalted butter in a large pot over medium heat. Add minced garlic and sauté for about 1 minute.

Sprinkle 1/4 cup of all-purpose flour into the melted garlic butter, whisking continuously for 1–2 minutes to form a roux.

Gradually whisk in 2 cups of heavy cream and cook for about 5 minutes until thickened. Stir in 1/2 cup Parmesan cheese, thyme, salt, and pepper.

Add the drained potato slices to the creamy sauce, gently folding until well-coated.

Layer the coated potato slices vertically in the greased baking dish and pour any remaining sauce over the top. Sprinkle additional Parmesan cheese on top.

Cover with aluminum foil and bake for 30 minutes. Uncover and continue baking for another 30 minutes until golden brown and bubbling.

Remove from the oven and let rest for about 10 minutes before serving.