Heat oven to 400° F. In a small bowl, combine the parsley and lemon zest. Set aside.
Season the beef with 1½ teaspoons salt and ½ teaspoon pepper. Heat 1 tablespoon of the oil in an ovenproof skillet over medium-high heat. Brown the beef on all sides, about 5 minutes.
Transfer the beef 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 the potatoes until tender, about 15 minutes. Drain, divide among plates, and gently crush with a spoon to crack the skins.
Drizzle the potatoes with the remaining 3 tablespoons of oil, ¼ teaspoon each salt and pepper, and Parmesan. Serve with the beef. Top with the parsley.