How to Make It
Peel and cut sweet potatoes into 1/8-inch-thick slices, using a mandoline or a knife. Bring a large pot of water to a boil over high. Add sweet potato slices, and cook until slightly softened, 3 to 4 minutes. Drain and spread in a single layer on a paper towel-lined baking sheet. Let potatoes stand until cool and dry, about 20 minutes. Arrange slices, standing vertically on edges, in a lightly greased 13- x 9-inch broiler proof baking dish.
Bring cream to a simmer in a medium saucepan over medium. Whisk in black pepper, 2 tablespoons of the brown sugar, and 1 teaspoon of the salt. Cook, stirring occasionally, until thickened slightly, about 10 minutes. Pour over potatoes. Cover and chill until ready to bake, up to 1 day ahead.
Stir together flour, butter, remaining 2 tablespoons brown sugar, and remaining 1/2 teaspoon salt in a small bowl. Stir in hazelnuts, and set aside.
Preheat oven to 350°F. Remove casserole from refrigerator while oven preheats. Bake, covered, 45 minutes. Uncover and top evenly with hazelnut mixture. Return to oven, and bake 15 minutes. Increase oven temperature to broil, and broil until topping is golden brown, about 5 minutes. Let stand 10 minutes before serving.