1. In a large pot of salted boiling water, cook spaghetti until al dente, 8 to 10 minutes. Drain, reserving 1 cup pasta water, and return to pot.

2. Place lemon zest and juice in a small bowl. Thinly slice remaining 1?2 lemon into half moons and remove seeds, then set aside.

3. Add lemon juice and zest, butter, Parmesan, and 1/2 cup pasta water to pot, then 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 parmesan.