Preheat oven to 400 degrees Fahrenheit.

Prick the sweet potatoes all over with a fork and place on a baking sheet. Roast for 45 to 50 minutes, or until fully cooked. Remove from oven, cut the sweet potatoes in half, and allow to cool.

Once the sweet potatoes have cooled, scoop them out and place the flesh into a bowl. Add the ricotta, Parmesan, salt, pepper, nutmeg, and egg and blend using an immersion blender or whisk. Add the flour in small quantities and blend until just combined. Chill the dough by refrigerating for at least two hours or freezing for 45 minutes.

Generously flour a work surface. Cut the dough into four sections and shape each one into a long rope about one inch thick. Add more flour as needed to prevent the dough from sticking.

Cut the dough into one-inch squares. Gently roll each square against the tines of a floured fork, to form ridges, then place on a floured baking sheet. Repeat with the rest of the dough.

Bring a large pot of salted water to a boil and cook the gnocchi for two to three minutes. (Gnocchi are ready when they float to the top.)

In a large skillet, melt the butter over medium heat. Once sizzling, add the garlic and sage and cook for one to two minutes. Stir in the gnocchi and stir to coat in butter. Season with salt and pepper.

Transfer the gnocchi to a plate and garnish with grated or shaved Parmesan. Serve hot.