Instructions

In a bowl, combine all the topping ingredients and set aside.

To prepare the gratin: Preheat the oven to 350 degrees. Butter a shallow 2-quart gratin dish with 1/2 teaspoon of the butter.

In a small saucepan, melt the remaining 2 tablespoons butter 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 both potatoes. 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, Parmesan and shallot cream. Mix gently but thoroughly and scrape into the prepared gratin dish, smoothing and pressing until evenly distributed. Cover with the bread crumb 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. Remove from oven and let rest for 15 minutes before serving.