Peel the Yukon gold potatoes and slice them into thin, uniform rounds, about 1/8 inch thick, using a mandolin. Place them in a bowl of cold water.

In a medium saucepan, combine heavy cream, grated Parmesan cheese, fresh thyme, salt, and pepper over medium heat. Stir continuously for 5-7 minutes until the sauce thickens.

Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter. Layer the sliced potatoes vertically in the dish, pouring a portion of the cream sauce over each layer.

Cover the baking dish with aluminum foil and bake for 45 minutes. Remove the foil and bake uncovered for an additional 15 minutes until the top is golden and bubbly.

Allow the dish to rest for about 10-15 minutes before serving.