sweet potato-russet potato gratin

Ingredients For sweet potato-russet potato gratin

• topping:
• 1 1/2 c

  fresh bread crumbs
• 2 Tbsp

  unsalted butter, melted
• kosher salt
• 1 Tbsp

  dijon mustard
• 2 Tbsp

  parmigiano-reggiano, grated
• 2 tsp

  fresh flat-leaf parsley, chopped
• gratin:
• 2 Tbsp

  unsalted, butter
• 1 c

  shallots, thinly sliced
• kosher salt
• 2/3 c

  heavy cream
• 2/3 c

  chicken broth
• 2 Tbsp

  prepared horseradish
• freshly ground black pepper
• 1

  russet potato, peeled and cut in half lengthwise, thin sliced half moons
• 1

  sweet potato, peeled cut in half lengthwise, thin sliced half moons
• 1/3 c

  parmigiano-reggiano, grated