Mash the potatoes with the butter and season with salt, pepper and nutmeg. Sprinkle with flour, add the egg and mix, forming a dough. Knead with hands.
Place the dough on a surface sprinkled with flour, cut in 4 pieces and roll each one of them to form a cylinder of about 2/4 inch thick. Cut them in pieces of 1 inch each.
Roll each gnocchi on a fork to give it its characteristic shape. The lines that will form on them have the purpose of holding the sauce better. Put a large pot with water on high heat and when it boils, lower the heat to medium.
Cook the gnocchi in small portions in the boiling water, to avoid them from sticking to one another. Let them cook for 3 to 5 minutes. You will know they are done when they float to the surface.
As they float, remove them from the water and let them rest on a tray lined with parchment paper. Keep them warm while you cook the rest.
Serve gnocchi immediately with your favorite pasta sauce.