Finish - Add 1/2 cup of pasta water to the sauce and bring to a simmer. Scoop the pasta and the remaining butter into the sauce. Reduce the heat to low, and fold in the cheeses, tossing until melted. Add more pasta water if the sauce is too tight. Remove from the heat, add the arugula, and toss until the arugula is just wilted and everything is coated with the sauce. Garnish with lemon zest and parsley.