Boil the potatoes until just tender. Test them by piercing them with the fork. Drain and leave overnight
Next day, peel and grate them coarsely and season with salt and pepper. Heat half the butter and oil in a heavy frying pan and press the potatoes in to make a cake
Cook over moderate heat for about 20 minutes or until the bottom is golden and crusty
Carefully invert the rosti on to a plate. Heat the rest of the butter and oil in the pan, slide the rosti back into the pan and cook the second side for 10 more minutes or until golden and crusty