1.Preheat the oven to 180°C.
2.Rub the potato with a little olive and salt and pop into the oven for 75-90 mins or until very tender. Pull out the oven and let it cool enough to handle. Slice in half and scoop out the flesh. Mash using a ricer or with a fork in a bowl, be as thorough as you can!
3.Tip the mash out onto a clean surface and spread out a little. Sprinkle over the flour and season with a big pinch of salt. Start rubbing the flour through the potato with the tips of your fingers as if you’re making pastry. Once combined, drizzle over the egg yolk and briefly knead the mixture into a homogenous dough. Let rest for 15 mins.
4.Divide the dough in half, dust the surface with flour and roll each half of the dough out into a long sausage shape about 2cm thick. Cut the ropes into little pillows approximately 4 cm long and set aside.
5.Bring a medium pot of water to a boil and season with salt. Set a medium frying pan over a medium heat.
6.Drop the gnocchi into the water and cook for 2-3 mins until they float to the surface. Meanwhile, add a couple of glugs of oil to the frying pan. Transfer the gnocchi to the hot pan and cook for 1-2 mins on each side until golden brown and caramelised.
7.Now drop the butter into the frying pan and allow to foam and sizzle. Chuck in the sage leaves and let them crackle and fry for 30 seconds. Toss to coat the gnocchi and finish with big squeeze of lemon and Parmesan.
8.Tip onto a plate and get stuck in.