Cook the pasta according to package directions. Drain reserving 1/2 cup of the cooking water. Put back in pot with olive oil to prevent sticking.
Heat the oil in a large skillet. Add the leeks and cook stirring until they begin to soften, 4 min. Stir in the garlic and sage and cook 2 min. Add the sweet potato, salt, and pepper and cook until just tender, 8-10 min.
Stir in 3/4 cup of the Parmesan and the reserved cooking water and simmer, stirring until the potatoes are fully tender, 4-6 min.
Add the vegetables and nutmeg to the pasta pot and toss.
Sprinkle servings with the remaining Parmesan.