Steam the potato for 10 minutes or until soft then allow to cool for 10 minutes.
Put the potato through a potato ricer or blend in a food processor until smooth then transfer to a large bowl.
Add the egg to the potato with a big pinch of salt and mix until just combined.
Add the flour and mix again until combined. Do not kneed as this will make your gnocchi chewy and dense instead of fluffy. The dough will feel wet, sticky and hard to work with but any Italian will tell you that the less flour you use the better so although it will feel messy, collect the dough up into a ball, dust in a layer of flour and continue with the recipe.
Dust your bench/large chopping board with more flour then place the dough on the floured surface. Halve the dough using a bench scraper (or knife).
Roll out/shape the dough into a long piece about 1 inch wide. Use the bench scraper to cut up the dough into small pieces of gnocchi.
Bring a pot of water to the boil with a big pinch of salt, add the gnocchi in two batches. Boil for 2 - 3 minutes, the gnocchi is ready when it floats to the top. Transfer the cooked gnocchi to a bowl.
Bring a large pan to medium heat with the butter. Add the garlic and sauté for 1 - 2 minutes then add the sage and fry until it starts to crisp up.
Add the gnocchi to the pan and turn up the heat, to high. Pan fry the gnocchi until browned and crispy. Season with salt and pepper.
Transfer to serving bowls, shave some fresh parmesan on top and enjoy!