Poke a few holes in your sweet potatoes with a fork or knife, then place them on a microwave-safe plate. Microwave the sweet potatoes until they're easily pierced with a knife, initially checking them after five minutes, then every one to two minutes after that (Note 1).
Start your water for boiling the gnocchi. Place a large pot of water on the stove over high heat. You can turn the heat down if it boils before your gnocchi are ready, but you'll want it to be at a rolling boil once they're done.
Cut the sweet potatoes open and let them cool a bit while you gather your other ingredients.
Once the sweet potatoes are cool enough to handle, scoop out the flesh and transfer it to a large mixing bowl. Use a fork or potato masher to mash the sweet potatoes (Note 2). You should end up with about two cups of mashed sweet potato.
Mash the salt and nutmeg into the sweet potato, then begin adding flour, about a half cup at a time. Keep adding flour until a soft dough forms, adding between 1 ½ and 2 cups total. Be careful not to overwork the dough.
Lightly flour a work surface, then divide the dough into three or four portions. Roll each dough portion into a 1-inch thick snake.
Cut each dough snake into ½-inch pieces. You'll get around forty pieces. Optionally, use a fork to crimp each of your gnocchi.
Once all of the gnocchi are made, carefully transfer them to the boiling water. Boil the gnocchi until they float, which should only take about three or four minutes. Drain them into a colander, or you can remove them individually with a slotted spoon as they're done.
Melt the vegan butter in a large skillet over medium heat. Add the sage leaves and walnuts. Fry the sage and walnuts for about a minute, stirring occasionally.
Add the gnocchi to the skillet, placing the dumplings in an even layer. Continue to fry everything for three to four minutes, stirring occasionally, until the gnocchi start to crisp up. Keep an eye on everything to watch for burning. If you start to see the sage or walnuts burning, the dish is done (even if you can't see browning on the gnocchi). Remove it from heat immediately.
Season the dish with salt and pepper to taste. Serve.