Wash the potatoes and boil in a pan of salted water for about 30 minutes.
Drain, peel, press through a potato ricer and allow to cool.
Add the egg and flour to the potatoes. Season with salt and a pinch of nutmeg.
Mix everything quickly to a smooth dough.
Place the dough on a floured surface and shape into finger-sized rolls; cut each roll in 1.5cm wide pieces.
Gently press each piece on both sides with a fork. Cover with a tea towel and leave to stand for about 10 minutes.
Add to a pan of boiling salted water.
Cook for about 8 minutes, but no longer, stirring occasionally.
The gnocchi are done when they float to the surface of the water. Stir occasionally.
Melt the butter in a hot pan and fry the sage leaves.
Remove the gnocchi from the pan with a slotted spoon, pour into the hot butter and simmer briefly. Season with salt and pepper and sprinkle with Parmesan.