Heat oven to 450º F. Place the beef in a roasting pan and rub the oil, cracked pepper, salt, and garlic all over the meat to coat evenly. Place the beef in the oven and roast for about 20 minutes. Turn over and roast another 25 minutes or until an instant-read thermometer inserted in the center of the roast reads 150º F. Remove from oven and let rest, covered, for 10 minutes. Reserve a quarter of the beef for another use.
While the beef is cooking, place the potatoes in a large pot and cover with lightly salted water. Bring to a boil over high heat. Cover, reduce heat to low, and simmer until the potatoes are tender, about 15 minutes. Drain. Reserve 1 cup of potatoes for another use. Mash the remaining potatoes with a fork until smooth. Mix in the remaining ingredients.
Thinly slice the beef and serve with the mashed potatoes.