In a small bowl, combine the parsley and lemon zest; set aside.
Season the beef with 1 1/2 teaspoons salt and 1/2 teaspoon pepper.
Heat 1 tablespoon of the oil in an ovenproof skillet over medium-high heat. Add the beef and brown on all sides, about 5 minutes. Transfer to oven and roast, 18 to 20 minutes for medium-rare (internal temperature 125° F). Let rest for 5 minutes before slicing.
Meanwhile, place the potatoes in a saucepan, cover with cold water, and bring to a simmer. Add 1 teaspoon salt and cook until tender, about 15 minutes. Drain and divide among plates. Gently crush the potatoes with a spoon to crack the skins. Drizzle with the remaining oil, 1/4 teaspoon salt, 1/4 teaspoon pepper, and the Parmesan. Serve with the beef. Top with the parsley.