Mix pumpkin puree with egg yolk. Season to taste with salt and nutmeg. Quickly stir in flour with a fork; it will still be fairly sticky. The more flour you add, the denser the finished product will be.
Flour your board generously, and dust a parchment-lined baking sheet generously with flour too. Take a lump of dough the size of a large walnut, roll into a finger-wide log, and quickly cut into gnocchi about 1" long. Dip the knife blade in flour to minimize sticking. Transfer to prepared baking sheet while making the rest.
Bring a large pot of water to a boil. Add a generous pinch of salt. Turn heat down so water bubbles gently. Add gnocchi. Stir once so they don't stick to the bottom, then cook until they float, 4-6 minutes depending on size. Remove from water with a skimmer; dumping them through a colander might damage them,.
Stack and shred sage leaves into fine chiffonade. Melt butter over medium heat, add sage, and saute until the sage crisps and the butter turns brown and smells nutty (don't let it burn!). Spoon over gnocchi, sprinkle with parmesan cheese and ground black pepper.
Optional: start with a small whole sugar pumpkin, 1½-2 lbs. Clean out seeds and fibers. Slice into 1/4" thick slices and roast in one layer on a baking sheet in a preheated 400°F oven for 30 minutes or until soft. Press through a potato ricer or food mill. Let cool 15 minutes before proceeding.