2 firm ripe pears (about 1/2 lb each), peeled, cored, and sliced crosswise into 1/8-inch-thick rounds
1/4 cup grated Parmesan cheese
Preheat oven to 375°F. Mist an 8 x 10-inch baking dish with cooking spray.
In a medium saucepan on medium, heat oil; add leeks and garlic and cook, stirring occasionally, until softened, about 5 minutes. Stir in milk, thyme, nutmeg, salt and pepper, and bring to a boil (still on medium). Remove from heat.
Reserving most of milk mixture, line base of dish with leeks using a slotted spoon. Arrange alternating slices of celery root, potatoes and pears, starting with edges of dish then filling center with slices to fit.
Pour milk mixture evenly over top. Loosely cover with foil and bake for 1 hour. Remove cover, sprinkle with cheese and continue baking until tender and golden brown, 20 to 30 minutes more. Let stand for 5 minutes before serving.