Preheat the oven to 500 degrees F. Rub the roast with black pepper. Place it in a roasting pan (don't use a rack) fat side up and surround with the onion pieces. Cook for 20 minutes, then reduce the oven temperature to 350 degrees F and remove the roast from oven. Carefully transfer the meat to a plate.
To the pan, add the potatoes to the side, return the meat to the pan, and put it all in the oven for 50 minutes, or until a meat thermometer inserted into the roasts thickest part reads 155 degrees F. Remove the roast from the oven and let stand for 20 minutes before carving.