Preheat the oven to 425ºF. Pierce each potato using a fork. Then, wrap the potatoes in aluminum foil and place them on a baking dish. Bake in the preheated oven for 40-60 minutes or until ready. Check the readiness using a fork, if potatoes are fork tender - remove them from the oven and let cool completely. Then, refrigerate overnight or for at least 3-4 hours.
When ready to make potatoes in the casserole, preheat the oven to 350ºF. Remove potatoes from the refrigerator, grate, and place in a large bowl.
Then, add minced shallots, 2 cups grated white cheddar cheese, sour cream, garlic powder, salt, and freshly ground white pepper. Mix until completely combined.
Grease a 9x13 inch casserole dish with non-stick cooking spray. Then, transfer potatoes Romanoff with sour cream to the dish. Top with the remaining 1 cup grated white cheddar cheese and grated parmesan cheese. Bake in the preheated oven for 25-35 minutes or until golden brown. Enjoy!