Ingredient

• 2 pounds sweet potatoes (about 5 small), peeled and cut into 1/2-inch pieces
• 2 pounds russet potatoes (about 7 small), peeled and cut into 1/2-inch pieces
• Coarse salt and freshly ground pepper
• 3/4 cup whole milk, warmed
• 3/4 cup heavy cream, warmed
• 1 1/2 sticks unsalted butter, melted
• 2 tablespoons all-purpose flour
• Pinch of cayenne pepper
• 1 leek, white and light-green parts only, cut into 1/4-inch rounds, washed well, and dried (about 2 cups)
• Safflower oil, for frying

• Step 1

  Preheat oven to 425 degrees. Cover sweet potatoes and russet potatoes each with 1 inch water in 2 separate medium saucepans. Bring to a boil, salt generously, and boil until fork-tender, about 8 minutes. Drain and return potatoes to respective pans.

• Step 2

  In a bowl, combine milk, cream, and butter. Divide milk mixture between saucepans. Mash potatoes until smooth; season with salt and pepper.

• Step 3

  Spread mashed sweet potatoes evenly in the bottom of a shallow 2-quart baking dish. Top with mashed russet potatoes; spread evenly to edges. Bake until golden, about 25 minutes.

• Step 4

  Meanwhile, in a bowl, combine flour and cayenne. Add leeks; toss to coat. In a medium saucepan, heat 2 inches oil over medium-high heat until a thermometer registers 350 degrees. Working in batches, add leeks and cook, turning, until crisp and light golden around edges, about 1 minute. With a slotted spoon, transfer to paper towels to drain. Season with salt.

• Step 5

  Sprinkle fried leeks over potatoes; serve.