2 tablespoons olive oil
2 medium leeks, white and light green parts sliced lengthwise and thinly sliced crosswise
Salt and freshly ground pepper
1 tablespoon fresh thyme leaves
1/2 cup heavy cream
1 large sweet potato (about 1 pound), peeled and sliced into 1/2-inch-thick rounds
1 large or 2 medium red skinned apples, such as Gala or Honeycrisp (about 12 ounces), halved, cored and cut into 1/4-inch slices
1/4 cup freshly grated Parmesan