Preparation

• 在th机架位置e center of the oven and heat the oven to 400°F.

  Prick the potatoes a few times with a fork. Put the potatoes directly on the oven rack and bake until tender when pierced with a skewer, 50 to 60 minutes.

  While the potatoes are still hot, hold each one with a clean dishtowel and cut off about one-quarter lengthwise. With a spoon, scoop the potato flesh out into a medium bowl, leaving enough on the skins that they hold their shape. Add 4 Tbs. of the butter to the potato flesh, and with a potato masher, work the potatoes until lightly mashed but not completely smooth. Stir in 1/2 cup of the pecorino, the sour cream, half-and-half, 2 Tbs. of the horseradish, the chives, 1 tsp. salt, and a few grinds of pepper and mix until combined. Mound the filling into the potatoes. Cut the remaining 1 Tbs. butter into 4 pieces and top each potato with a pat of butter. Transfer to a small rimmed baking sheet or baking dish. (The potatoes may be prepared to this point up to 6 hours ahead; cover and refrigerate. Add 5 to 10 minutes to the baking time.)

  In a small bowl, thoroughly mix the remaining 2 Tbs. pecorino and 2 tsp. horseradish with your fingers. Sprinkle over the potatoes. Bake until the potatoes are heated through and the tops are golden-brown, about 20 minutes. Sprinkle with chives.