Adding sweet potatoes to a gnocchi recipe makes it a little easier for the home cook to pull off, because sweet potatoes have less starch than potatoes, and a high starch content can make gnocchi dense and tough. The combination of sage and sweet potatoes is one for the ages.
Preheat oven to 375ºF. Place sweet potatoes and potatoes on a baking sheet and roast until soft, about 2 hours. Let cool slightly, then cut in half and scoop the flesh into a large bowl. (This should yield about 5 lbs of roasted potato mixture.)
Add flour, salt and pepper to the potato mixture and mix together until smooth. Make a well in the centre and pour in eggs. Using a fork and starting in the centre of the mixture, incorporate eggs into mixture.
Turn out onto a lightly floured surface and knead until a soft, slightly sticky, spongy dough forms, being careful not to work dough too much. Shape into a ball and place on a lightly floured cutting board. Cut ball into 8 pieces and cover with a clean tea towel. Dust a baking sheet with flour.
On a lightly floured surface, working with one piece of dough at a time and keeping the rest covered, roll each piece into a 20-inch rope, about 1/2 inch thick. Cut rope into 1-inch lengths. Using your thumb, roll each piece of dough over the back of the tines of a floured fork, leaving an indention from your thumb on one side and the markings from the fork on the other. Place gnocchi on prepared baking sheet. Repeat with remaining dough. (Make ahead: Cover and refrigerate for up to 1 day, or freeze in an airtight container for up to 1 month.)
Meanwhile, bring a large pot of salted water to a boil over high heat. Plunge half the gnocchi into the boiling water. Once they float to the surface, cook for 1 minute longer. Using a slotted spoon, transfer to a baking sheet or plate and continue cooking the remaining gnocchi. Drizzle with oil and toss to coat. Sprinkle with Parmesan and fried sage leaves.