Sprinkle roast with salt and pepper and place, fat side up, on a rack in a shallow roasting pan.
Bake at 325°, basting occasionally with pan drippings, for about 2 hours and 45 minutes, until a meat thermometer inserted into thickest portion registers 145° (medium-rare), or to desired degree of doneness. Remove roast from pan, reserve drippings; let stand 20 minutes before slicing.
Pour drippings into a large saucepan; skim fat. Add shallots and cook over medium-high heat, stirring occasionally, until reduced by half. Whisk in port wine.
Whisk together flour and broths; whisk into wine mixture. Bring to a boil; reduce heat, and simmer, whisking often, 15 minutes or until thickened. Stir in peppercorns; serve with roast. Garnish, if desired.