Potatoes Romanoff is a super simple mashed potato casserole fancy enough for the upcoming holidays, yet easy enough as a side for weekly family dinners. It's creamy and cheesy and can be made ahead when you're rushed for time!
Preheat oven to 425 degrees. Wrap each potato in tinfoil. Pierce each potato a few times with a fork. Place potatoes on oven rack and bake until tender, about one hour. Remove from oven and remove foil. Let cool to room temperature. Place the potatoes on a plate and wrap each potato in plastic wrap. Refrigerate overnight.
Preheat oven to 350 degrees. Grate potatoes with the large holes of a box grater, including the skins. Transfer to a large bowl.
In a small saute pan, melt the butter over medium-high heat. Add the chopped shallots and cook until translucent, stirring occasionally, about 5 minutes. Add to the grated potatoes with 1 3/4 cups of the grated cheese and salt and pepper. Gently toss the mixture with your hands. Fold in the sour cream in two additions.
Transfer the mixture to a 1 1/2 quart gratin dish. Do not compress the mixture. Sprinkle with the remaining cheese. Bake until the potatoes are hot and the cheese is golden brown, about 30 minutes.