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1
Add the water to a large saucepan and set on the stove over high heat to come to a boil. When it's very hot, add the 1 1/2 tablespoons kosher salt (this will probably happen while you're working on the scallion sauce).
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2
While the water is heating up, prep the scallions: Wash well and trim the hairy ends-but keep each bunch separate. Chop one bunch of scallions into large (roughly 1 1/2-inch) pieces. Chop another into medium (about 1/2-inch) pieces. And mince the remaining bunch into oblivion (I like to do this by chopping each scallion as finely as possible, then running my knife through the pile a few times).
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3
Make the scallion sauce: Heat a large cast-iron skillet over medium-high heat. When it's very hot, add 1 tablespoon oil. When the oil is shimmery, add the large-sized scallion pieces. Toss a few times and sprinkle with salt. Now don't move the pieces for 3 minutes, until each begins to char.
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4
Add the medium-sized scallion pieces to the pan. Toss and season with another pinch of salt. Lower the heat to medium and cook for another 3 minutes, stirring once or twice, until soft.
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5
Add the minced scallions to the pan. Toss and season with another pinch of salt. Lower the heat to the lowest possible setting, then add the remaining 4 tablespoons olive oil. Cook for about 2 minutes, stirring once or twice, then cut the heat. Stir in the red pepper flakes, if you're using them.
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6
Your water is probably boiling at this point. Add the spaghetti. After a moment, gently nudge it down with a wooden spoon as needed, until all the noodles are submerged. Boil for about 9 minutes or until just al dente.
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7
Right before the spaghetti is done, turn on the heat under the scallion sauce to the lowest setting. Use tongs to transfer the spaghetti to the pan with the scallions. Add 1/4 cup pasta water and toss, toss, toss to form an emulsified sauce. Add more water if necessary (I usually add another 2 tablespoons). This should only take a minute or so-take care not to cook the pasta too long.
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8
Season with more salt to taste and sprinkle more red pepper flakes on top, if you want. Serve immediately.
