Heat oven to 425° F. On a rimmed baking sheet, toss the potatoes, oil, garlic, ¼ teaspoon salt, and ¼ teaspoon pepper. Roast until golden brown and tender, 35 to 40 minutes. Toss with the arugula.
Fifteen minutes before the potatoes are done, heat an ovenproof skillet over medium-high heat. Season the steak with ½ teaspoon each salt and pepper and brown, 3 minutes per side.
Transfer the skillet to oven (on the rack above the potatoes) and cook for 6 to 8 minutes for medium-rare. Let rest for 5 minutes before slicing. Serve with the potatoes and arugula.