The day before, boil the peeled potatoes in initially cold water, draining them when they are not yet fully cooked (they must remain a bit firm).
Let them cool and put them in the refrigerator.
When preparing the rosti, peel the potatoes and pass them on a grater with large holes to obtain strips.
Chop the onion.
Heat oil and butter in a heavy-bottomed non-stick pan, add the potatoes, salt, pepper and mix making them flavor well in the seasoning then mash them with the back of a spoon forming a compact pie.
Keep the flame at half height and cook for about ten minutes, occasionally moving the pan.
It will take about ten minutes before a golden and crispy crust forms; at this point, gently slide the rosti onto a plate, cover it with the pan then firmly grasp the plate and the pan and turn everything upside down with a quick gesture.
Brown this side also in the same way and serve the rosti.