Combine butter, Worcestershire sauce, and vinegar in a saucepan. Cook over low heat until butter melts; stir often. Remove from heat; keep warm.
Make 6 small slits in tenderloin; stuff each slit with a garlic clove. Place tenderloin on a lightly greased rack in a shallow roasting pan. Tuck narrow end under to make roast more uniformly thick. Insert meat thermometer, if desired. Bake, uncovered, at 350° for 40 minutes or until meat thermometer registers 140° (rare), basting often with prepared sauce.
Remove and discard garlic. Cut tenderloin into 1/2-inchthick slices; arrange on a warm serving platter. Garnish with parsley sprigs, if desired; serve immediately.