SWEET POTATO-RUSSET POTATO GRATIN WITH HORSERADISH & A DIJON CRUST

For the topping:

1 1/2 cups coarse breadcrumbs (from an airy, crusty loaf like ciabatta)

2 tablespoons unsalted butter, melted

Kosher salt

1 tablespoon Dijon mustard

2 tablespoons grated Parmigiano-Reggiano

2 teaspoons flat-leaf parsley, chopped

For the gratin:

2 tablespoons plus 1/2 teaspoon unsalted butter

1 cup (6 or 7) shallots, thinly sliced

Kosher salt

2/3 cup heavy cream

2/3 cup low-salt chicken broth

2 tablespoons horseradish

Freshly ground black pepper

1 (12-ounce) russet potato

1 (12-ounce) sweet potato

1/3 cup grated Parmigiano-Reggiano

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 degrees. Rub a shallow 2-quart gratin dish with 1/2 teaspoon of the butter.

Melt the remaining 2 tablespoons 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.

Peel the potato and sweet potato, cut each in half lengthwise, and slice each across into thin half-moons. In a large bowl, combine them along with 2 teaspoons salt, the Parmigiano, and 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 have subsided, and the potatoes are tender when pierced with a fork, about 1 hour. Let rest for 15 to 20 minutes before serving.

Serves 8.