Sweet Potato-Russet Potato Gratin

• For the Topping
• 11/2 cups coarse fresh bread crumbs (from an airy crusty loaf like ciabatta)
• 2 Tbs unsalted butter, melted
• 1 Tbs Dijon Mustard
• 2 Tbs grated Parmesan
• 2 tsp chopped fresh parsley
• Kosher Salt
• For the Gratin
• 2 Tbs unsalted butter, plus 1/2 tsp for the baking dish
• 1 cup thinly sliced shallots
• Kosher salt
• 2/3 cup heavy cream
• 2/3 cup homemade or low salt chicken broth
• 2 Tbsp prepared horseradish
• Fresh black pepper
• 1 Russet potato (12 oz)
• 1 Sweet potato (12 oz)
• 1/3 cup grated parmesan

1. Make the topping
2. In a bowl combine the breadcrumbs, melted butter, and a pinch of salt. Mix in the mustard and then the Parmesan and parsley
3. Prepare the Gratin
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.
5. Assemble the gratin
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)
7. Bake the Gratin
8. Cover with the breadcrumb topping
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

bread crumbs, butter, mustard, parmesan, parsley, kosher salt, gratin, butter, shallots, kosher salt, heavy cream, chicken broth, horseradish, fresh black pepper, potato, sweet potato, parmesan

Taken from www.epicurious.com/recipes/member/views/sweet-potato-russet-potato-gratin-50043888 (may not work)