Heat oven to 375 F. In a roasting pan, toss the onions, oil, 1/2 teaspoon salt, and 1/2 teaspoon pepper. Push the onions to the edges of the pan. Season the beef with 1 teaspoon each salt and pepper and place in the center of the pan. Roast the beef to the desired doneness, 65 to 80 minutes for medium-rare (remove from oven when the internal temperature registers 125 F).
Transfer the beef to a cutting board, tent loosely with foil, and let rest for at least 15 minutes. Slice and serve with the onions.
