For the pasta:
For the sauce:
1. Make a pasta dough: On a baking tray covered with parchment paper bake sweet potato until tender (about 40 minutes) at 400f/205c. Let cool 10 minutes, peel, transfer to a bowl and mash well.
2. To the bowl add ricotta cheese, grated parmesan cheese, salt, pepper, lemon zest and freshly ground nutmeg. Mix well and transfer to a piping bag.
3. Divide the pasta dough into two-four pieces. Roll out the first piece into at least 0.5inch/1cm before using pasta machine (keep remaining pieces covered with plastic wrap so it doesn’t dry out).
4. Dust the pasta machine and dough with flour.
5. Set machine to #0 and feed it through. Fold one side of the piece of dough into the middle, then fold the other side over that to form 3 layers and feed it through the machine again, still on #0. Repeat at least 5 times.
6. Switch setting to #1 and feed it through the machine once. Switch setting to #2 and feed it through the machine again. Do the same until #5 or #6 if you like your pasta very thin.
7. Cut strips of rolled out dough so that they are about 1-inch longer on both sides of the ravioli mold. Place one sheet of pasta over the dusted ravioli maker. *Gently* press the pasta with plastic form. Pipe the ravioli. Place another sheet of pasta over the ravioli filling. Use a rolling pin to press the two layers of pasta together. Do it gently. Turn the ravioli maker over and give the mold a good shake until they fall out of the mold.
8. Bring a large pot of salted water to a boil over high heat.
9. When the water are boiling start with the sauce, Place the butter in a skillet next to the pot, melt over medium-high heat, when the butter is melted toss the sage in and add the ravioli into the boiling water for 2-3 minutes. Ladle in about 3-5 tbsp of cooking water and bring to a boil. Cook until reduced by about half. Add the ravioli to the skillet and toss to coat, about 1-2 minutes.
10. Divide the ravioli among plates (5-8 ravioli per person) together with the sauce and toss with parmesan.