From Blue Hill Restaurant
7 tablespoons cold butter
1 1⁄2 pounds apples, (about 5) peeled, cored, quartered, and sliced 1⁄4-inch thick
1 cup half and half
2 teaspoons salt
1 teaspoon white pepper
1⁄2 teaspoon cinnamon
Big pinch ground nutmeg
2 1⁄2 pounds sweet potatoes, peeled, cut into 1⁄4-inch-thick rounds
1⁄2 cup panko or fresh bread crumbs
Preheat oven to 350°. In a 12-inch cast iron skillet, melt 4 tablespoons butter over medium heat. Add the apples, and sauté in batches until lightly browned but still firm. Set the apples aside on a plate. In a large bowl, stir together the half and half, salt, pepper, cinnamon, and nutmeg. Toss the potatoes in the cream to coat.
Generously butter an 8 x 8 baking pan. Line the bottom of the dish with a layer of overlapping potatoes, followed by overlapping apples. Repeat, finishing with a layer of potatoes. Pour in remaining spice-cream mixture.
Cover with aluminum foil, and bake for about 1 1⁄2 hours, or until the potatoes are cooked. Heat the broiler. Sprinkle panko over the gratin, dot with 1⁄2-inch cubes of the remaining butter, and broil until the bread crumbs are golden brown.