Place beef tongue in a deep pan with salt, allspice, onion, and water to cover.
Bring water to a boil, then reduce heat and simmer until the tongue is fork-tender. (Allow about 1 hour per pound.) Cool tongue in liquid, remove, and cut off the bones and gristle at the thick end.
Slit the skin on the underside, starting at the thick end; loosen the skin with a sharp paring knife, then peel and pull it off, working from the thick end to the tip.
Chill and slice thinly.
Remove surface fat and connective tissue from the beef tenderloin; place the meat on a rack in a roasting pan.
Tuck the narrow end of the tenderloin under to make a uniform thickness.
Melt butter and blend with garlic; brush meat with the butter mixture.
Roast the tenderloin in a 450° oven for 30 minutes for rare meat or 40 minutes for medium; baste occasionally with garlic butter.