Peel the potatoes and grate them, using the large holes of a hand grater or a food processor. 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, working with a fistful at a time, squeeze as much liquid as possible out of them and transfer to a second bowl. (The potatoes will start to discolor, but that won’t really affect the final results.)
Large holes mean faster work, better texture. A very finely grated potato could turn mushy during cooking.
You need to get rid of as much liquid as possible, so just squeeze small handfuls at a time.
To make one large rösti—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 about 1/8 inch; add more if-necessary.) When the oil begins to ripple and quiver slightly, test it by dropping in a potato shred—it should sizzle enthusiastically. If not, wait a few more seconds. When the temperature is right, take a fistful of potatoes, wring it out once more, and let it fall loosely from your fingers into the center of the pan. (Be careful because the oil will spatter; getting hit by a few tiny droplets is inevitable.)
Fill the pan gradually. Adding just a small amount at a time makes it easier to get an even layer.
Working quickly, repeat until you’ve got enough potatoes in the pan to cover the bottom. With a fork, gently spread out the shreds of potato to make a layer about 1/2 inch thick, trying to distribute them evenly, avoiding dense or thin patches. If there are straggly potatoes around the edges, tuck them in with the fork also so they don’t burn.
Adjust the heat so that you hear a lively sizzle but the bottom isn’t browning too rapidly. Cook until the underside is a deep golden brown and the potatoes on the top are starting to look translucent, 12 to 16 minutes. (Taste a few strands—they should be almost fully cooked and tender.)
If you’re confident enough to just flip the rösti in the pan, go for it—it’s quick and efficient. If you’re not, carefully slide the rösti out of the pan onto a dinner plate and return the pan to the heat. Put another plate on top of the rösti and, holding tightly, flip the plates over. Slide the inverted rösti back into the pan and continue cooking until the new bottom is browned and the potatoes feel really tender in the middle when poked with a knife (try to snitch a few strands from the center and taste them for doneness), another 6 to 8 minutes. Slide the rösti onto a cutting board if you’re serving it immediately, or to a cooling rack to hold it for a few minutes. Blot the top with a paper towel to remove any excess oil. Cut into wedges and serve as soon as possible.