Preheat your oven to 375°F (190°C). Wash, peel, and thinly slice about 3-4 medium russet potatoes, aiming for 1/8-inch thick rounds. Layer the sliced potatoes in a greased 9x13-inch baking dish, sprinkling salt between layers.

In a medium saucepan, combine 1 cup of heavy cream and 1 cup of grated Gruyère cheese over medium heat. Stir until the cheese is melted and the mixture is smooth, about 3-4 minutes. Add garlic powder, black pepper, and fresh thyme.

Pour the cheese sauce evenly over the layered potatoes, ensuring each slice is coated. Sprinkle grated Parmesan cheese on top for extra flavor. Optionally, add breadcrumbs for a crunchy topping.

Cover the baking dish with aluminum foil and bake for 40 minutes. Then remove the foil and bake for an additional 15-20 minutes until the top is golden and bubbling.

Remove from the oven and let it rest for about 10 minutes. This helps the layers set.

Carefully cut the gratin into squares to serve. This dish pairs well with roasted meats or fresh salads.