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1
Position a rack in the upper third of the oven and preheat to 450 degrees F. Peel the sweet potatoes and slice into 1/8-inch-thick rounds (use a mandoline if you have one).
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2
Peel half of the russet potatoes and slice into 1/8-inch-thick rounds.
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3
(Do not rinse-you'll need the starch for the dish to set.)
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4
Coat the bottom of a 10-inch nonstick ovenproof skillet with 4 tablespoons melted butter.
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5
Starting in the center of the skillet, arrange a layer of russet potato slices in concentric circles, slightly overlapping them.
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6
Sprinkle with a pinch of salt and some of the rosemary leaves.
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7
Set the skillet over medium heat.
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8
Working quickly, add the remaining sliced russets in 2 more layers, then the sliced sweet potatoes in 2 layers, sprinkling lightly with more salt and rosemary between layers.
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9
(Check to make sure the bottom is browning evenly; adjust the heat as needed.)
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10
Drizzle with 4 tablespoons melted butter.
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11
Peel and thinly slice the remaining russet potatoes and add them to the skillet in 2 or 3 more layers, lightly seasoning each layer with salt and rosemary.
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12
Drizzle with the remaining 4 tablespoons melted butter.
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13
When the potatoes are golden brown on the bottom, transfer the skillet to the oven and bake until the potatoes soften slightly, about 30 minutes.
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14
Remove from the oven; center the bottom of a 9-inch cake pan on the surface of the potatoes and press firmly a few times to help the layers stick together.
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15
Remove the cake pan and return the skillet to the oven; bake until the potatoes are tender and the top is golden brown, about 20 more minutes.
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16
Let the potatoes cool 5 minutes in the skillet, then run a small knife around the edge to loosen.
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17
Set an inverted serving plate on top of the skillet and carefully flip the potatoes onto the plate.
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18
Season with salt and pepper.
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19
Photograph by David Malosh
