Boil potatoes in their skins. Drain and allow to stand until cool enough to handle. Scrape skin from potato with paring knife. Press them through a potato ricer. Spread the riced potatoes into a thin, even layer on the work surface and allow to cool completely.
In a small bowl, beat together the egg, salt, pepper and nutmeg. gather cold potatoes into a mound and form a well in the center. Pour egg mixture into the potato well. Knead the potato and egg mixture together with both hands, gradually adding the grated cheese and enough of the flour, about 1 1/2 cups, to form a smooth but slightly sticky dough. It should take no longer than 3 minutes to work the flour into the potato mixture; remember, the longer the dough is kneaded, the more flour it will require, and the heavier it will become. As you knead, repeatedly rub dough from hands and scrape it from the work surface back into the dough.
On a flour-dusted surface, dust dough and cut it into 6 equal pieces. Form dough into a rope. Slice rope into 1/2-inch thick rounds. Sprinkle rounds with flour and roll them into balls. Hold the tines of a fork at a 45-degree angle to the table with the concave part facing up. Dip the tip of your thumb in flour. Take one ball of dough and with the tip of your thumb, press the dough lightly against the tines of the fork as you roll it downward toward the tip of the tines. As the dough wraps around the tip of your thumb, it will form into a dumpling. Set on a baking sheet lined with a floured kitchen towel. Repeat with remaining dough. At this point, gnocchi must be cooked immediately or frozen.
To cook gnocchi, drop into salted boiling water. Cook, stirring gently until tender, about 1 minute after they rise to the surface of the pot.