Lay out a clean teatowel. Grate the squash, potato and apple together and spread out over the teatowel. Season generously and scatter over the sage. Gather up the teatowel and squeeze out as much liquid as you can into a sink. The more juice you get out, the crisper it will be. Add a dash of oil to the pan and tip the grated veg into the pan. Squish down into the pan with a spatula and continue to cook for five minutes. After five minutes add a couple of knobs of butter to the edges of the pan. Using a large plate and a dash of confidence flip the rosti out and over. Tidy up the edges with your spatula and transfer to the oven to cook for fifteen minutes.