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1
Preheat the oven to 400.
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2
Coarsely grate 4 of the potatoes into a colander.
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3
Toss with 1 tablespoon of kosher salt and let drain for 5 minutes.
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4
Using your hands, squeeze out as much liquid as possible from the potatoes and transfer to a bowl.
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5
Grate the apple and add to the potatoes.
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6
Add the onion, 1 tablespoon of the olive oil and the sage and toss well.
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7
Season with pepper.
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8
Using a sharp knife or a mandoline, slice the remaining potato 1/4 inch thick.
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9
In a 10-inch nonstick ovenproof skillet, heat 1 tablespoon of the olive oil.
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10
Arrange the potato slices in the skillet in concentric circles.
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11
Season with salt and pepper and cook over moderate heat, pressing down on the potatoes with a spatula a few times, until the slices brown around the edges, about 5 minutes.
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12
Spoon the grated potato mixture into the skillet, pressing firmly to make a neat cake.
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13
Transfer the skillet to the oven and bake for about 35 minutes, or until browned and crisp on the bottom.
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14
Invert the cake onto a flat plate.
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15
Add the remaining 1/2 tablespoon of olive oil to the skillet.
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16
Slide the potato cake back into the skillet and cook over moderately low heat until nicely browned on the bottom, about 5 minutes.
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17
Slide the cake onto a cutting board and garnish with the chives.
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18
Cut into 4 wedges and serve on plates with the Horseradish Cream.
