6 tablespoons unsalted butter, room temperature, plus more for dish
5 pounds Yukon Gold potatoes, scrubbed
1/3 cup sour cream
4 ounces cream cheese, room temperature
6 ounces raclette or fontina, grated (2 1/2 cups)
1/2 cup whole milk, room temperature
Kosher salt and freshly ground pepper
1/3 cup extra-virgin olive oil
2 teaspoons minced garlic (from 2 cloves)
16 small whole fresh sage leaves, or 8 large leaves, coarsely chopped
2 tablespoons finely chopped fresh chives, for serving
Preheat oven to 375 degrees, with racks in upper and lower thirds. Butter a 2- to 2 1/2-quart baking dish. Bake potatoes directly on upper rack until tender, 1 hour to 1 hour, 15 minutes. Remove and let cool slightly; when cool enough to handle, cut in half and scoop out flesh, reserving skins. Rice or mash potato flesh in a large bowl; add butter, sour cream, cream cheese, half of raclette, milk, 2 teaspoons salt, and 1/4 teaspoon pepper and mix until smooth and creamy. Transfer to prepared dish; top with remaining raclette.
Slice potato skins into 1 1/2-inch-wide strips. Transfer to a rimmed baking sheet; toss with oil, garlic, sage, and 1/2 teaspoon salt. Spread in a single layer. Bake casserole on upper rack and skins on lower rack until cheese melts and skins become crisp and golden brown, 25 to 30 minutes. Sprinkle roasted skins over casserole; top with chives and serve.
The potatoes can be made through step 1, covered, and refrigerated for up to three days; store the skins and flesh separately. Bring to room temperature an hour before baking, and increase the time by 10 minutes.