4 cups heavy cream
3 Tbs. kosher salt
1 Tbs. fresh thyme leaves
15 white potatoes, peeled (about 7 1/2 lbs.)
6 cups shredded Gruyère cheese
Fresh thyme, for garnish
Preheat oven to 350°F.
Heat the cream, salt, and thyme leaves in a medium saucepan over medium-low heat.
Slice potatoes 1/16th-inch thick or less, preferably with a mandoline. Layer them in a 9- by 13-inch baking dish, overlapping, until they reach 3/4 inch from the top edge.
Stir the cream to be sure that the salt has completely dissolved, then pour it over the potatoes. Cover the baking dish with foil and bake the potatoes for 75 to 90 minutes, or until a toothpick meets no resistance when inserted in the middle.
Spread the cheese on top and place the potatoes back in the oven, uncovered. Continue to bake until the cheese has fully melted, but not browned, about 10 minutes.
Allow to rest for 10 minutes. (The dish can be pre-scored if an exact number of servings is crucial.) Garnish with sprigs of fresh thyme and serve.