Preheat oven to 450°F.
Sprinkle roast lightly with salt. Place the roast, fat-side up, on rack in roasting pan. Roast 10 minutes. Reduce oven temperature to 250°F. Roast until oven thermometer reads 125°F to 130°F for medium-rare, or 135°F to 145°F for medium, 20 to 30 minutes per pound. (The temperature will continue to rise 5° out of the oven.)
Meanwhile, in 10-inch skillet over medium heat, melt butter. Add onion and shallots. Cook, stirring occasionally until onions and shallots are tender, about 10 minutes. Stir in brown sugar, balsamic vinegar and TABASCO® Sauce; cook 2 minutes longer until mixture has thickened slightly.
After 1 hour of roasting beef, brush meat with onion mixture. (Cover meat with foil if top gets too brown.)
Remove meat to platter. Cover loosely with foil; let stand 15 minutes before carving.
Makes 8 servings.