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1
Make the topping
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2
In a bowl combine the breadcrumbs, melted butter, and a pinch of salt. Mix in the mustard and then the Parmesan and parsley
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3
Prepare the Gratin
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4
Heat the oven to 350F. Rub a shallow 2 qt. gratin dish with 1/2 tsp butter. Melt the remaining2 Tbs butter in a small saucepan over medium heat. Add the shallots and a big pinch of salt, cook, stirring often 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. Remove from heat.
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5
Assemble the gratin
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6
Peel the potatoes, cut each in half lengthwise, and slice across into thin half moons. In a large bowl combine the sweet and russet potato slices, 2 tsp salt, the Parmesan 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. (if assembling ahead, stop now and refrigerate; bring it close to room temperature before baking)
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7
Bake the Gratin
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8
Cover with the breadcrumb topping
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9
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 15 to 20 minutes before serving
