Preheat your oven to 400 degrees.
Poke a few holes in each potato, then bake them in the oven for 45 minutes to an hour. The flesh inside should be completely tender. Cut the potatoes in half and let some steam escape. When they’re cool enough to handle scoop the potato meat into a bowl. Toss the skins or reserve them for another use (like stuffed potato skins).
Pass the scooped potato through a ricer, then spread them in a thin layer out on a flat work surface. Evenly distribute the flour, parmesan, salt and nutmeg on top. Then whisk the egg and drizzle that all over the potatoes.
Carefully begin to incorporate the ingredients using a fork or a bench scraper. When the ingredients are mostly incorporated finish the job with your hands, but be careful not to over knead the dough. Stop just as the dough comes together and it’s a cohesive mass.
On a clean and floured surface cut off some dough and begin to roll it out into a ½ inch diameter log. Cut the log into 1-inch pieces. Roll each dumpling down a gnocchi board or down the back of a fork.
Bring salted water up to a light simmer, then drop the gnocchi in and cook until they float to the surface. This usually takes about a minute. Work in batches so you don't crowd the pot.
You can use the gnocchi right away or spread them out on a sheet and cool them off in the refrigerator. Once chilled, place the gnocchi in a container. They'll keep for about a week in the fridge.
To re-heat, just add them directly to a saute pan with all of you other ingredients.