Peel the potatoes and grate them, using the large holes of a hand grater.
Put the potatoes in a large bowl, add the salt and pepper, and toss to coat thoroughly. Let the potatoes rest for at least 5 minutes, and then, taking one handful at a time, squeeze as much liquid as possible out of them and transfer to a second bowl.
Do not worry if the potatoes begin to oxidize, this is normal.
Heat a heavy-based skillet that measures about 8 inches across the base over medium-high heat. Add the oil (it should come to a depth of at least 1/8 inch) and half the clarified butter or ghee. When the oil begins to shimmer just before it reaches its smoking point, add in the potatoes and press down with a fork or spatula to fill the pan. Please note that they will sizzle so don't let it startle you. If you want to make individual servings, you can use 4 small 4" cast iron skillets like I did.
Keep a watch on the rosti and when the bottom is browned you need to flip it over. It is easiest to do this with a flat cutting board or dinner plate. Once you flip it, add more oil and butter to the pan, when hot slide back in the rosti and cook for another 10 minutes until golden brown.
Slide the rösti onto a cutting board, Blot the top with a paper towel to remove any excess oil. Cut into wedges and serve as soon as possible.