Preheat the oven to 220° C.

Pierce the sweet potatoes with a fork, place in a baking pan, and bake until fully cooked, about 45 to 60 minutes. Remove from oven and let sit until cool enough to handle.

Using a sharp knife, cut the potatoes in half lengthwise. Using a large spoon, scoop the flesh out of each sweet potato into a medium bowl. Thoroughly mash the sweet potatoes while they are still warm, then set aside or refrigerate to cool completely.

Add salt, nutmeg, and pepper to the sweet potatoes. Add flour, ½ cup at a time, mixing well with a spoon to combine. Once a soft, slightly sticky dough has formed, divide it into six portions.

Generously flour the work surface and your hands. Roll each portion of dough into ropes about 1.25cm in diameter. Each rope will be approximately 18 to 23cm long.

Dip a sharp knife in flour and cut each rope into 2.5cm long pillows. If desired, roll each pillow on fork tines to make decorative ridges.

Fill a medium saucepan with heavily salted water and bring it to a boil. In the meantime, heat the margarine and sage in a large nonstick skillet until the margarine begins to bubble.

When the water is boiling, reduce the heat to a gentle simmer and gently drop in the gnocchi, about 20 at a time. The gnocchi will float to the surface in about 4 minutes. Continue to cook about 30 seconds more.

Using a slotted spoon, immediately transfer the gnocchi to the skillet of butter sauce. Let cook, turning frequently, for 1 to 2 minutes. You will have to do this in several batches, until all the gnocchi are cooked.

Serve immediately, topped with additional freshly ground black pepper.

Note: Uncooked gnocchi can be made in advance and kept frozen for up to 1 month or refrigerated for 3 to 4 days until ready to boil.