1 1/2 cups heavy cream
1 sprig fresh thyme
2 pounds russet potatoes, peeled and cut into 1/8-inch thick slices
Salt and freshly ground black pepper
1/2 cup grated Parmesan, plus more for broiling
2 1/2 cups Gruyere or Swiss cheese, grated
Preheat oven to 375 degrees F.
In a saucepan, heat up the cream with a sprig of thyme.
While cream is heating up, butter a casserole dish. Place a layer of potato in an overlapping pattern and season with salt and pepper. Remove cream from heat, then pour a little over the potatoes. Top with some grated Parmesan and Gruyere. Make 2 more layers ending with a last layer of cheese. Bake, uncovered, for 45 minutes. Sprinkle some more Parmesan and Gruyere and broil until cheese browns, about 5 minutes.