A healthier version of a classic potato gratin made with leeks for extra flavor and topped with grated Parmesan.
2 tablespoons extra-virgin olive oil, divided
2 medium leeks, white and tender green parts only, thinly sliced crosswise
Coarse salt and ground pepper
1 Tablespoon finely minced fresh sage (or 1 teaspoon dried)
¼ teaspoon freshly grated nutmeg
2 pounds sweet potatoes, peeled and sliced ⅛-inch thick
½ cup freshly grated Parmesan cheese