Arrange a rack in the middle of the oven and heat the oven to 425ºF. Scrub and wrap the potatoes individually in aluminum foil. Pierce each potato all over with a fork. Place the potatoes directly on the oven rack and bake until knife-tender, about 1 hour.

Let cool to room temperature, then refrigerate still wrapped in foil for at least 8 hours or overnight.

Heat the oven again to 425ºF. Finely chop the shallot and garlic.

Melt the butter in a medium frying pan over medium heat. Add the shallots and cook until softened, 3 to 4 minutes. Add the garlic and cook until very tender, 1 to 2 minutes more. Scrape into a large bowl.

Unwrap the potatoes and grate them unpeeled on the large holes of a box grater into the bowl. Tear any large pieces of the potato skin up into smaller pieces.

Grate the cheese and add 1 1/2 cups of the cheese to the bowl, reserving the remaining for topping.

Add the sour cream, kosher salt, and black pepper to the potatoes and gently stir together with a spatula until well-combined.

Transfer the mixture into a 7 or 8x11-inch or other 2-quart baking dish and spread into an even layer.

Sprinkle with the reserved cheese. Bake until heated through, golden-brown, and the cheese on top is bubbling, about 30 minutes.

If the top isn’t as browned as desired, broil on high for 2 to 3 minutes. Let cool for 15 minutes before serving.