Cook the pasta according to the package directions. Drain, reserving 1/2 cup of the cooking water.
Heat the oil in a large skillet over medium heat while the pasta cooks. Add the leeks and cook, stirring, until they begin to soften, 4 minutes. Stir in the garlic and sage and cook 2 minutes. Add the sweet potatoes, 1/2 teaspoon salt, and 1/2 teaspoon pepper and cook, covered, stirring twice, until just tender, 6 to 8 minutes. Stir in 1/2 cup of the Parmesan and the reserved cooking water and simmer, stirring, until the potatoes are fully tender, 2 to 4 minutes. Add the cooked pasta and nutmeg and toss.
Divide among individual bowls and sprinkle with the remaining Parmesan and 1/2 teaspoon pepper.
Upgrade: For a heart-healthy boost and a bit of crunch, sprinkle in 1/2 cup chopped roasted almonds before serving. Try using butternut squash as a flavorful alternative to sweet potatoes.