Make the topping:
Combine the breadcrumbs, melted butter, and a pinch of salt in a bowl. Mix in the mustard and then the Parmigiano and parsley.
Prepare the gratin:
Heat the oven to 350°F. Rub a shallow 2-quart gratin dish with 1/2 tsp. of the butter. Melt the 2 Tbs. butter in a small saucepan over medium heat. Add the shallots and a big pinch of salt; cook, stirring frequently, until softened, limp, and somewhat golden. Whisk together the cream, broth, and horseradish; add to the shallots. Season with salt and pepper, stir to combine, and take off the heat.
Assemble and bake the gratin:
Peel the potato and sweet potato, cut each in half lengthwise, and slice each across into thin half moons. In a large bowl, combine the potato and sweet potato slices, 2 teaspoons salt, the Parmigiano, and the shallot cream. With a rubber spatula, mix gently but thoroughly and scrape into the prepared gratin dish, smoothing and pressing until evenly distributed.Cover with the breadcrumb topping.
Bake until the crust is deep golden brown, the juices around the edges are have subsided, and the potatoes are tender when pierced with a fork, about 1 hour. Let rest for 15 to 20 minutes before serving.
nutrition information (per serving):
based on eight servings;
Photo: Scott Phillips