Cook the pasta according to the package directions. Drain, reserving ½ cup of the cooking water.
Heat the oil in a large skillet over medium heat. Add the leeks and cook, stirring, until they begin to soften, 4 minutes.
Stir in the garlic and sage and cook for 2 minutes.
Add the sweet potatoes and ½ teaspoon each salt and pepper and cook, covered, stirring twice, until just tender, 6 to 8 minutes.
Stir in ½ cup of the Parmesan and the reserved cooking water and simmer, stirring, until the potatoes are fully tender, 2 to 4 minutes.
Add the pasta and nutmeg and toss. Divide among individual bowls and sprinkle with the remaining Parmesan and ½ teaspoon pepper.