Place the roast, fat side up, on the rack in a shallow roasting pan. Generously rub the meat with salt and pepper.
Roast for 20 minutes. Reduce the heat to 325 F and roast for 20 minutes per pound, 1 1/4 to 2 1/2 hours. For medium-rare, the internal temperature should be about 120 F and medium will be about 135°F.
Remove the roast from the oven and let stand, covered loosely with aluminum foil, while making the gravy. Strain the pan drippings into a medium saucepan. Skim two-thirds of the fat off the top and discard.
Whisk the flour into the drippings until smooth. Cook on medium heat until thickened. Stir in the butter. Season with salt and pepper to taste.
Carve the roast and serve the gravy with the sliced meat.