Peel and slice 1/8-inch thick, 1 pound each of russet and sweet potatoes. Layer half of the potato slices in a greased 12-inch casserole, alternating between the sweet potato and russet potato. Top with about 2 ounces of grated Jarlsberg cheese, 1/2 teaspoon of fresh thyme leaves and 1/4 teaspoon salt. Repeat layering with remaining potato slices, topping the layer again with the same amount of cheese, thyme and salt as the first layer. Sprinkle pepper evenly over mixture and then pour 1 1/2 cups heavy cream over the top to completely cover. Bake in a 350 degree F. oven until mixture is golden and cheese is bubbly, about 1 hour 10 minutes. Let stand at least 5 minutes before serving.