Apple, Leek and Butternut Squash Gratin

extra-virgin olive oil

medium leeks, white part only, trimmed of roots and tough outer leaves, thinly sliced crosswise, well washed and dried

Coarse salt and ground pepper

dry sherry or apple cider

chopped fresh sage, plus leaves for garnish

butternut squash, peeled, seeded, and sliced 1/8-inch thick

apples, such as Gala, Cortland, Baldwin, or Macoun, peeled, halved, cored, and cut into 1/8-inch thick slices

freshly grated Parmesan cheese