Prepare the Cream Infusion

Preheat your oven to 325°F (165°C). In a small saucepan, combine the heavy cream, kosher salt, and minced garlic. Bring to a simmer over low heat and allow the garlic to infuse the cream for about 5 minutes. Once fragrant, remove from the heat and set aside.

Slice the Potatoes

Using a slicer or a sharp knife, thinly slice the Yukon Gold potatoes to about 1/8-inch thick rounds. Set the sliced potatoes aside in a bowl of cold water to prevent browning.

Caramelize the Shallots

In a cast iron skillet, melt the butter over low heat. Add the diced shallot and sauté gently, stirring occasionally, for 5 minutes, until the shallots are soft and slightly caramelized. Be careful not to burn them.

Layer the Potatoes

Pat the sliced potatoes dry with a paper towel. Begin arranging the potato slices in a spiral pattern in the skillet on top of the caramelized shallots. Overlap the slices slightly to create an even layer. Once the potatoes are arranged, pour the cream mixture evenly over them.

Bake the Gratin

Cover the skillet tightly with aluminum foil and place it in the preheated oven. Bake for 1 hour and 15 minutes, or until the potatoes are tender when pierced with a fork.

Crisp the Cheese

After baking, remove the skillet from the oven and preheat the broiler to high. Uncover the skillet, and sprinkle the grated Gruyère and Parmesan cheeses evenly over the potatoes. Sprinkle the fresh thyme on top for a burst of flavor. Return the skillet to the oven under the broiler for 5 minutes, or until the cheese is golden brown and crispy.

Serve and Enjoy

Once the cheese is bubbly and browned, remove the gratin from the oven. Allow it to cool slightly before serving, and enjoy the perfect balance of creamy, cheesy goodness with a crispy finish.