1. Grate the potatoes on the large holes of a box grater.
2. Place the grated potatoes in the middle of a waffle towel or a paper towel, gather up the corners and drain off the liquid from the potatoes.
3. Place the potatoes in a bowl, add melted butter, salt and pepper. Mix well.
4. In a large, nonstick skillet heat the oil over medium-high heat. Arrange the mixture in the skillet so that it covers the bottom.
5. Cook over medium heat for about 5-7 minutes, until the side is golden brown. Shake the skillet to loose the rosti and turn it over using a wide spatula.
6. Cook for another 5-7 minutes until golden brown.
7. Slide the rosti on the cutting board, cut into portions and serve.