Spaghetti with Lemon, Arugula, and Prosciutto

Bring a large pot of salted water to boil. Cook spaghetti until al dente, 8 to 10 minutes. Drain, reserving 1 cup of cooking liquid, and return to pot. Zest and juice 1 lemon and place in a small bowl. Thinly slice remaining 1/2 lemon into half moons and remove seeds. Set aside. Add lemon juice and zest, butter, Parmesan, and 1/2 cup pasta water to pot. Stir gently to combine, adding additional pasta water until creamy. Add red pepper flakes, prosciutto, arugula, and lemon slices and toss until arugula just begins to wilt. Season with salt and pepper, garnish with Parm, and serve.