To bake the potatoes, pierce all over with a fork and wrap in aluminum foil. Bake the potatoes for 1 hour at 425F, then refrigerate still wrapped overnight. This is also a great use for leftover baked potatoes!
To make the casserole, very gently grate the refrigerated baked potatoes, skin and all, on a box grater.
In a shallow dish or large bowl, combine the grated potatoes, onion, salt, several grinds of pepper, cheese, and sour cream. Mix very gently – I like to use my fingers to do this. The idea is to mix the ingredients without mushing or breaking the potato pieces.
Transfer the potato mixture to a baking dish, heaping the mixture to mound in the center of the casserole. Pour the melted butter over the Potatoes Romanoff and sprinkle with a bit more salt and pepper. This can be done in advance!
When ready to bake, preheat the oven to 375F. Bake the Potatoes Romanoff uncovered for about 30 minutes until hot all the way through and lightly browned on top. YUM.