Mix coarse salt, pepper, and allspice in bowl. Sprinkle spice mixture over roast. Let stand at room temperature 1 hour.

Preheat oven to 375°. Heat oil in large wide ovenproof skillet over medium-high heat until nearly smoking. Add roast, fat side down, to skillet; cook until well browned on bottom, 3 to 4 minutes. Turn roast, fat side up, and transfer skillet to oven. Roast until instant-read thermometer inserted into center of meat registers 130° for medium-rare, about 50 minutes. Transfer to platter; let rest while preparing sauce.

Pour off all but 2 tablespoons drippings from skillet and heat over medium-high heat. Add shallots to skillet and saute until soft, about 3 minutes. Add Madeira; boil 1 minute. Add broth and boil until liquid is reduced by about 1/3, stirring occasionally, about 8 minutes. Add any accumulated juices from meat to skillet; boil 1 minutes longer. Turn off heat; add frozen butter and swirl skillet until butter is blended into sauce. Season with salt and pepper.