Cook the pasta and when it’s done, reserve one cup of pasta water, drain the pasta and set it aside.
Add to the empty pasta pot the lemon juice, zest, butter, cheese and pasta water, a little at a time, until the sauce has a creamy consistency. Add the pasta, prosciutto and hot pepper flakes, if using.
Add the arugula and stir until the arugula is slightly wilted. Salt and pepper to taste. Serve and garnish with a little more Parmesan. (If you really love lemon, squeeze a little on at the end to brighten the taste.)