12 ounces multigrain penne
4 teaspoons olive oil
3 leeks (white and light green parts), cut into half-moons
3 cloves garlic, thinly sliced
1/4 cup fresh sage, chopped
2 small sweet potatoes (about 1 pound), peeled and cut into 1/2-inch pieces
kosher salt and pepper
3/4 cup grated Parmesan
1/4 teaspoon ground nutmeg
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. 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 potato, 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.