When Leonard Bernstein entered the pit at La Scala Opera House, Milan on December 10, 1953, he became the first American ever to conduct at that theater. His star singer that evening was Maria Callas, and their performance of Cherubini’s Medea was quickly hailed a landmark of operatic history. The central role of Medea, a sorceress who kills the children she has borne to her faithless husband Jason, is hugely demanding, but a searingly on-form Callas redefined its possibilities in spectacular fashion. In the Act I finale she seethes with vengeful intentions, while at the opera’s murderous conclusion Medea’s complex swirl of seething, sometimes conflicting emotions is viscerally depicted. Bernstein, who learned the opera at short notice, injects a fiery energy into a score which had long been neglected, sustaining the dramatic charge at a high voltage throughout. The sound has limitations, but the performance itself is unmissable.