Start by preheating your oven to 400°F (200°C). Grease a medium-sized casserole dish with 1 tablespoon of butter, ensuring all sides are coated to prevent sticking, and set it aside.

Wash and peel the potatoes, then use a mandoline slicer to cut them into thin, even rounds about 2mm thick. Place the sliced potatoes in a large bowl and set aside while you prepare the other ingredients.

Heat a large skillet over medium heat. Add the bacon and cook until it’s crispy and golden brown, stirring occasionally to ensure even cooking. Remove the bacon from the skillet and chop it into small pieces. In the same skillet, add the chopped onion to the bacon fat and sauté until softened and lightly caramelized, about 5 minutes. Combine the bacon and onions together and set aside.

Grate the Gruyère, Classic Cheese, and Parmesan cheeses using a box grater or food processor. In a medium bowl, mix the three cheeses together, reserving a small handful for the topping later.

In a large mixing bowl, combine the heavy cream with the roasted garlic (squeezed out of its skins), thyme leaves, salt, and freshly cracked black pepper. Add the bacon and onion mixture, and stir until everything is evenly incorporated.

Add the sliced potatoes to the cream mixture and gently toss to coat each slice with the creamy sauce. Layer the coated potato slices into the prepared casserole dish, ensuring they are slightly overlapping for even cooking. Pour any remaining cream mixture over the top, spreading it evenly.

Cover the casserole dish tightly with aluminum foil and place it in the preheated oven. Bake for 1 hour, allowing the potatoes to cook through and absorb the creamy sauce.

After an hour, remove the foil. Return the dish to the oven and bake uncovered for an additional 10-15 minutes, or until the top is golden brown and bubbling.

Remove the dish from the oven and let it rest for 10 minutes to allow the flavors to meld. Sprinkle freshly chopped chives over the top for a pop of color and flavor before serving.