Yield: 6–8 servings | Prep time: 20 minutes | Cook time: 60 minutes
- 1 1/4 to 1 1/2 pounds celery root, peeled, halved, and sliced 1/8 inch thick
- 1 large russet potato (weighing about 1 pound), peeled, halved, and sliced 1/8 inch thick
- 1 large, slightly under-ripe pear, peeled (if desired), cored, and sliced 1/8 inch thick
- Coarse salt and freshly ground pepper
- 1 1/2 cups crème fraîche
- 6 ounces grated Gruyère cheese
Preheat oven to 400°F and generously butter a medium baking dish. Layer half of the celery root, potato, and pear slices in the pan. Generously salt and pepper this layer. Stir the crème fraîche to loosen the consistency, then pour half over the top. Spread it to the edges of the pan with a flexible spatula. Distribute half the Gruyère over the crème fraîche.
Layer the remaining slices of celery root, potato, and pear over the cheese, overlapping them as necessary to fit in your dish. Generously salt and pepper this layer, then cover with the rest of the crème fraîche and Gruyère.
Bake the gratin uncovered for 20 minutes. Then cover with aluminum foil and bake about 20 minutes more. Check for doneness with a paring knife; you should feel almost no resistance when you insert the knife into the gratin. Remove the aluminum foil and continue to bake until the top is deep golden brown, 10–15 minutes longer. Remove the gratin from the oven and let rest 10–15 minutes before serving.