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in a large pot of salted boiling water, cook spaghetti until al dente, 8 to 10 minutes.
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drain, reserving 1 cup pasta water, and return to pot.
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place lemon zest and juice in a small bowl.
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thinly slice remaining ½ lemon into half-moons and remove seeds, then set aside.
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add lemon juice and zest, butter, parmesan, and ½ cup pasta water to pot, then stir gently to combine, adding additional pasta water until creamy.
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add red pepper flakes, prosciutto, arugula, and lemon slices and toss until arugula just begins to wilt.
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season with salt and pepper, garnish with parm, and serve.