Smooth butter over the top of the roast and sprinkle with salt and pepper. Let stand at room temperature for an hour or more.
Preheat oven to 350°F. Sprinkle with chopped onions and place roast on rack in uncovered roasting pan. Roast for 1 1/4 hours, if you wish it rare, basting with pan drippings from time to time. Add 15 minutes cooking time for medium, another 15 minutes for well done. Allow the meat to rest for a few minutes before carving. Transfer to a warmed serving platter.