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Preheat the oven to 400 degrees F.
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Scrub the potatoes well and dry with paper towels.
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Poke several holes in them with a skewer to allow steam to escape.
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Place the potatoes on a small baking sheet and bake, uncovered, until tender, about 1 hour.
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Allow the potatoes to cool to room temperature.
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Refrigerate in a plastic bag with a resealable closure overnight.
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Peel the potatoes and cut into 1/3-inch dice.
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Melt the butter in a large skillet over medium heat and add the onion.
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Cook, stirring often, until softened, about 5 minutes.
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Season with salt and pepper.
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Use a slotted spoon to transfer the onion to a small bowl.
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Add the vegetable oil to the skillet and heat until almost smoking.
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Add the potatoes and leave to form a crisp crust.
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Do not disturb the potatoes until the crust is formed, about 5 minutes.
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Check to see if a crust has formed; if so, give a quick stir.
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If not, continue cooking for another 1 to 2 minutes, until the crust has formed.
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With a spatula, turn over large chunks of the crusted potatoes.
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Season with salt and pepper and continue cooking until golden brown.
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Stir in the onion, season again with salt and pepper, and stir until the hash is heated through, about 3 minutes.
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Serve hot.
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**Cook time does not include refrigerating the cooled potatoes overnight.