Unlike most gratins, this potato and root vegetable version contains no milk, cream, or cheese. Instead, it gets its deep flavor from caramelized onions, chicken stock and, of course, butter. As the gratin bakes in the oven, the vegetables absorb the stock and develop a creamy texture while the top becomes browned and crispy.
Preheat the oven to 375 degrees F. Melt 3 tablespoons butter in a large cast-iron skillet over medium-high heat. Add the onion, thyme, 1/2 teaspoon salt and a few grinds of pepper. Cook, stirring occasionally, until the onion is dark brown in spots, 6 to 8 minutes. Reduce the heat to medium-low and continue cooking until soft and completely browned, 10 to 15 more minutes. Remove to a bowl and discard the thyme sprigs. Reserve the skillet.
Meanwhile, peel the potatoes, rutabaga and celery root and slice into 1/8-inch-thick rounds using a mandoline or a chef’s knife. (If any of the vegetables are very large, cut them in half first.) Cut the remaining 5 tablespoons butter into small pea-size pieces.
Arrange the celery root slices in the cast-iron skillet in layers, sprinkling each layer with salt and 4 or 5 pieces of butter. Spread half of the browned onion on top. Layer the rutabaga slices on top, sprinkling each layer with salt and 4 or 5 pieces of butter. Spread the remaining onion over the rutabaga. Layer the potato slices on top, sprinkling each layer with salt and 4 or 5 pieces of butter. Pour the chicken stock over the final potato layer, then dot with the remaining butter and season with salt and pepper.
Place the skillet over medium-high heat. When the stock begins to steam, transfer to the oven. Bake until the top is starting to brown and the stock bubbles up the sides, 50 to 60 minutes. Remove from the oven; tilt the skillet and spoon the stock over the potatoes. Continue baking until the vegetables are tender when pierced with a knife and just a little stock appears in the skillet when tilted, 45 to 60 more minutes. Let cool 10 minutes before serving.
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Photograph by David Malosh
Tools You May Need
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