Potluck Prep: The assembled gratin can be refrigerated overnight. To keep the nuts from getting soft, I like to sprinkle them on just before serving. While you can transport the gratin in an insulated carrier and serve it hot, it’s also terrific at room temperature.
- 2½ pounds sweet potatoes, peeled
- 1 large celery root (2 pounds), peeled with a sharp knife and halved lengthwise
- 1 medium rutabaga (2 pounds), peeled and halved lengthwise
- ½ tablespoon unsalted butter, melted, plus more for the pan
- kosher salt and black pepper, freshly ground
- 1 cup low-sodium chicken or vegetable broth
- ¼ cup heavy cream
- 1½ cups walnuts
- 1 teaspoon fresh thyme leaves, roughly chopped
- 1Using a mandoline or a very sharp knife, slice the potatoes lengthwise into slabs that are about ¹⁄8 inch thick. Halve the celery root and the rutabaga and slice each into ¹⁄8-inch-thick slabs.
- 2Preheat the oven to 375°F. Butter a 9-by-13-inch glass or ceramic baking dish.
- 3Arrange one-third of the sweet potatoes in the dish, overlapping them slightly; season with salt and pepper. Top with half the rutabaga, and then half the celery root, seasoning each layer. Repeat the layering, ending with a third layer of sweet potatoes. Pour the broth over and around the vegetables. Cover the baking dish tightly with foil.
- 4Bake for about 45 minutes, until the vegetables are almost tender when pierced. Remove the foil and pour the cream over the gratin. Bake for 30 minutes, until the liquid has thickened.
- 5Meanwhile, spread the walnuts on a baking sheet and toast in the oven for about 4 minutes, until fragrant. Let cool, and then coarsely chop. Return the chopped walnuts to the baking sheet, toss with the ½ tablespoon of butter and the thyme, and season with salt and pepper. Toast the nuts until nicely browned, about 2 minutes. Remove from the oven and top with the walnuts. Let stand for 10 minutes. Serve hot, warm, or at room temperature.