Place whole potatoes, in their skins on a baking tray and place them into the oven to cook for 1 hour until tender.
Leave to cool for 20 minutes and then cut in half and remove the skins from the potatoes. Pass the potatoes through a ricer then season with salt, white pepper and nutmeg.
Add 100g of flour and mix to form a dough. If it's too sticky, add a little more flour, but only a tablespoon at a time - too much makes the dough heavy.
Dust a tray with flour and cut the dough into 6. Working one piece at a time, roll the dough into a log about ½ inch thick.
Use a lightly floured knife or bench scraper to cut the logs into 1 inch pieces. Place on the lined tray, making sure the gnocchi don't touch each other.
Gently roll, 1 at a time, over the back of a lightly floured fork or butter pat -the ridges will help sauce cling to the gnocchi.
Bring a large saucepan of salted water to the boil and add one-quarter of the gnocchi. When they rise to the surface, count slowly to 10 and remove with a slotted spoon. Transfer to a baking tray, making sure there's a little cooking water still on the tray to stop them sticking together then set aside whilst you prepare the sauce.
Place the butter in a saucepan and cook until it turns golden then add the sage, lemon juice and a pinch of salt. Add the gnocchi to the pan and swirl to coat the gnocchi.
Serve immediately with a good grating of parmesan.