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1
Put oven rack in middle position and preheat oven to 375u00b0F.
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2
Prick potatoes in several places with a fork. Bake potatoes in a large shallow baking pan until tender when pierced with a sharp knife, about 1 1/2 hours.
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3
Transfer to a rack to cool completely, then chill, uncovered, at least 8 hours.
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4
Peel potatoes, then shred with slicer into a bowl. Coarsely grate onion on large teardrop-shaped holds of a box grater, then wrap in 2 layers of paper towels and squeeze gently to remove excess moisture. Toss together potatoes, onion, salt and pepper with a fork.
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5
Heat 2 tablespoons butter and 1 tablespoon oil in a 7- to 8-inch nonstick skillet over moderately high heat until foam subsides, then add potatoes, spreading them evenly and tamping them down with a rubber spatula.
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6
Reduce heat to moderate and cook Rosti until underside is a golden brown, about 5 minutes.
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7
Wearing oven mitts, invert a flat plate over skillet, then invert Rosti onto plate.
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8
Add remaining 2 tablespoons butter and remaining tablespoon oil to skillet and the until foam subsides, then slice Rosti (browned side up) back into skillet with robber spatula.
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9
Cook until underside is golden brown, about 5 minutes.
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10
Transfer to a cutting board and cut into 6 wedges.
