Heat oven to 400 degrees with a rack in the center position. Wrap each potato in foil and poke a few holes to allow the steam to escape. Bake until cooked through and a knife goes easily through a potato, about 1 1/2 hours. Set aside until cool enough to handle. Remove the foil, transfer potatoes to a plate, or similar dish, and let cool to room temperature, about 1 1/2 hours. Loosely cover with plastic wrap and refrigerate overnight to chill.

Heat oven to 375 degrees. In a kettle or a medium saucepan, bring 3 cups of water to a boil and keep at a slow simmer. Butter a 6-by-11-inch baking dish (or similar) and set aside. Set aside 1/2 cup of the cheese.

Using the large holes on a box grater and starting from the short end of the potato, grate the potatoes, with the skins on, into a large bowl; discard any large pieces of skin that don’t make it through the grater. Sprinkle in the salt and pepper, and, using your hands or 2 forks, gently toss the grated potatoes so they don’t get mushy and instead remain airy, light and fluffy. Taste and adjust seasoning with salt, being mindful that cheese will be added.

Add the shallot and the remaining cheese, and lightly toss again. Add the sour cream and gently toss until incorporated, taking care not to mash the potatoes.

Transfer the mixture to the prepared baking dish. Create a slight mound and gently spread, if necessary, without pushing the mixture down, then sprinkle on the reserved cheese. Place the dish on a rimmed sheet pan, transfer to the oven and very carefully pour the hot water into the sheet pan. Bake until browned in spots on top, 35 to 40 minutes.

Garnish with chopped chives, if you like, and serve.